Add support for AWS signature version 4 algorithm (#24)

* working v4 signatures without boto3 being used.

* removed debug messages

* expose AWS bad token errors

* tidy up code

* standardise on s3://<bucket>/<key> for mat for uri and enforce region in the s3auth.conf

* handle bucket errors when not in correct region

* updated README

* Remove erroneous newline from 301 and 400 error messages

* add test message instructions

* fixed default config used

* fixed default config used - to default /etc/apt/s3auth.conf

* ensure that metadata does not prevent region being set

* removed typo from README.md test message

* Need to url unquote the canonical uri before signing to create valid v4 signatures for the S3 API

* Need to canonical url quoted just for '+' before signing to create valid v4 signatures for the S3 API

* Adds retries for operations if they timeout. Delay before N retry is 2^N-1 (#17)

* added proxy configuration (#16)

* added apt configuration (for http proxy)

* typo

* doc fixes

* correct apt syntax in docs

* Use Acquire::http::Proxy as proxy config key

* Add support for path-based bucket name

On Python 2.7.12, this script fails with a `CertificateError` when a bucket name has a dot in it: `my.bucket.s3.amazonaws.com`. This change adds support for the alternative bucket URL, `s3.amazonaws.com/my.bucket/`

* Tidy bucket name finding.

* Fix merge conflicts.

* Merged upstream commits and resolved merge conflicts

* Added proxy instructions back in

README.md

@@ -0,0 +1,80 @@
+# apt-transport-s3
+
+### Table of Contents
+1. [License & Copyright](#license & copyright)
+2. [Requirements](#requirements)
+3. [Configuration](#configuration)
+4. [Usage](#usage)
+5. [Contribution](#contribution)
+
+## apt-transport-s3
+Allow to have a privately hosted apt repository on S3. Access keys are read from
+`/etc/apt/s3auth.conf` file or IAM role if machine is hosted on AWS or has
+access to AWS metadata server on 169.254.169.254.  They are also taken from the
+usual environment variables.
+
+## License & Copyright
+    # Copyright (C) 2014 Bashton Ltd.
+    #
+    # This program is free software; you can redistribute it and/or modify
+    # it under the terms of the GNU General Public License as published by
+    # the Free Software Foundation; either version 2 of the License, or
+    # (at your option) any later version.
+
+    # This program is distributed in the hope that it will be useful,
+    # but WITHOUT ANY WARRANTY; without even the implied warranty of
+    # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    # GNU General Public License for more details.
+
+    # You should have received a copy of the GNU General Public License
+    # along with this program; if not, write to the Free Software
+    # Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
+
+
+## Requirements
+### Additional package dependencies (except installed by default in Debian)
+1. python-configobj
+
+## Configuration
+/etc/apt/s3auth.conf or <a href="http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html">IAM role</a>
+can provide credentials required for using private apt repositories.
+
+NOTE: Region MUST match the region the buckets are stored in and if not defined defaults to us-east-1.
+
+Example of s3auth.conf file:
+```
+AccessKeyId = myaccesskey
+SecretAccessKey = mysecretaccesskey
+Region = 'us-east-1'
+```
+
+## Usage
+Install the .deb package from the releases page.  The bucket repo should be
+specified using an s3:// prefix, for example:
+
+`deb s3://aptbucketname/repo/ trusty main contrib non-free`
+
+if you need to use a proxy to connect to the internet you can specify this
+as an APT configuration directive (for example in
+/etc/apt/apt.conf.d/90apt-transport-s3)
+
+`Acquire::http::Proxy "http://myproxy:3128/";`
+
+## Testing
+The module will run in interactive mode.  It accepts on `stdin` and outputs on `stdout`.  The messages it accepts on stdin
+are in the following format and [documented here](http://www.fifi.org/doc/libapt-pkg-doc/method.html/index.html#abstract).
+
+```
+600 URI Acquire
+URI:s3://my-s3-repository/project-a/dists/trusty/main/binary-amd64/Packages
+Filename:Packages.downloaded
+Fail-Ignore:true
+Index-File:true
+
+```
+
+This message will trigger an s3 get from the above bucket and key and save it to Filename.  It needs a blank line after the message to trigger the processing by the s3 method.
+
+## Contribution
+If you want to contribute a patch via PR please create it against development
+branch. Patches via email are welcome as well.