Add AWS S3 backend

[laura-barluzzi]

This pull request adds a storage backend based on Amazon AWS S3 storage system. See #15

The AWS S3 storage system allows us to treat:

• Each paste as an Amazon S3 object
• Each past_id as an object key (or key name)
• Each metadata as an object metadata (which is accessible in a limited way, and which cannot be merely modified)

You find more information regarding Amazon S3 objects and their data, key, and metadata here.

Due to the lack of options, this PR does not implement any preventive approach for each communication with the Amazon S3 storage (e.g. setting a timeout).

This should be merged after #66 so that unrelated lint errors are fixed.

Pair-programmed with @c-w

[daknob]

Thanks for another great backend service!

[laura-barluzzi]

Hi @DaKnOb, @c-w and I finally fixed the merging conflicts for this PR and we also reduced redundancy of code by moving repeated decorator functions into utils.py.

This is ready to be merged!