Add backends for Gemstash storage of private gems

[gayashanbc]

Description

This pull request reflect the changes related to the GSoC project, "Add backends for Gemstash storage of private gems".

Blog Posts

1. Introduction & Implementing Support for The ‘backend’ Property
2. Implementing Trigger Function for AWS S3 Backend

[olleolleolle]

Tiny typo.

[olleolleolle]

This adds the first bit of functionality of the S3 backend feature.

That's good and will become useful to a user when gemstash offers an s3 backend.

(How do we want to proceed with building this feature?)

[indirect]

This is a good start! @gayashanbc, let's plan to continue adding commits to this PR, and we can give feedback and code review when you push or at least once a week. Once this PR has a more complete version of the backends feature, we can merge it.

[gayashanbc]

aws-sdk is added to the Gemfile.
I have tested and verified the ability to verify the SDK using either of the following options.

1. Default AWS credentials file (e.g. on Unix-like systems this file is located at ~/.aws/credentials)
2. Using environmental the environmental variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY

Currently, I'm working on displaying an error message when the SDK is not authorized using either of the above methods and displaying a link to official AWS page where it shows how the AWS credentials can be configured on the system. This will be implemented in the trigger action, when the backend is selected as 's3'.

[gayashanbc]

Above two commits implement the authorization trigger for s3 backend and the initial version of ask_s3_metadata method which checks if the given bucket_name already exists in AWS S3.

The next step is to create an S3 bucket using the given bucket_name if it does not exist already.

[gayashanbc]

Update: Above commit implements support for creating a new S3 bucket when the specified bucket is not owned by the user. It will also display an error when specified bucket_name is already taken by another user.

[bronzdoc]

closing this since a s3 backend was added in #270 thanks!