Code changes to implement proposed solution to GitHub issue #10

GitHub issue:
#10

See issue report and docstrings for detailed discussion of new
functionality.

Package changes:
* Added StrictAWS4Auth and PassiveAWS4Auth classes
* Added exceptions module with new exceptions: RequestsAWS4AuthException,
  DateMismatchError, NoSecretKeyError, DateFormatError
* Now only tested on Pythons 2.7 and 3.3 and up, to match Requests
  supported versions (dropped support for Python 3.2)

AWS4Auth changes:
* Added regenerate_signing_key() method, to allow regeneration of
  current signing key with parameter overrides
* Added methods for checking and extracting dates from requests:
  get_request_date(), parse_date(), handle_date_mismatch()
* __call__() now checks for a date header in the request and attempts
  to automatically regenerate the signing key with the request date if
  request date differs from the signing key date
* Can now supply a date to the constructor
* Changed default included sig headers to include 'date' header if
  present
* Docstring improvements

AWS4SigningKey changes:
* Added new store_secret_key instantiation parameter which allows
  control of whether the secret key is stored in the instance
* Deprecated the amz_date property in favour of just 'date'
* Spelling typo fix in AWS4AuthSigningKey module docstring. Thanks
  to jhgorrell

Test changes:
* Many new tests for the above functionality
* Added tests for generating canonical path, including test for fix
  added in 0.7 for percent encoding of paths
* Added tests for generating canonical querystrings

README.rst

@@ -49,7 +49,7 @@ Install via pip:
 
 requests-aws4auth requires the `Requests`_ library by Kenneth Reitz.
 
-requests-aws4auth is tested on Python 2.7 and 3.2 and up.
+requests-aws4auth is tested on Python 2.7 and 3.3 and up.
 
 Basic usage
 -----------

requests_aws4auth/__init__.py

@@ -41,7 +41,7 @@
 
 requests-aws4auth requires the Requests_ library by Kenneth Reitz.
 
-requests-aws4auth is tested on Python 2.7 and 3.2 and up.
+requests-aws4auth is tested on Python 2.7 and 3.3 and up.
 
 Basic usage
 -----------
@@ -143,9 +143,11 @@
 # http://opensource.org/licenses/MIT
 
 
-from .aws4auth import AWS4Auth
+from .aws4auth import AWS4Auth, StrictAWS4Auth, PassiveAWS4Auth
 from .aws4signingkey import AWS4SigningKey
+from .exceptions import RequestsAws4AuthException, DateMismatchError, NoSecretKeyError
 del aws4auth
 del aws4signingkey
+del exceptions
 
 __version__ = '0.7'