Allow S3 to be disabled

Introduce an environment variable AWS_DISABLE_STORE that, if set, causes
the `upload` function to just return an empty string and not try to send
to S3.

This is useful for development because;
- Developers don't need to keep S3 credentials around
- SSP can be developed on while offline
Merge pull request #186 from alphagov/allow-disable-s3

Allow S3 to be disabled