Introduce AWS S3 persister

[ottonemo]

This persister allows storing and loading models to and from AWS S3
buckets using s3fs to abstract the necessary API calls.

As discussed in #110 (comment) I have added the dependencies as extra dependencies and the imports are handled in the S3IO class.

[ottonemo]

Tests are currently failing since the moto library (a library to mock boto calls) is not installed by travis. Should we skip the tests or should we install these dependencies?

[alattner]

Thanks for the enhancement! Please add the dependencies to .travis.yml such that we get it tested in the CI pipeline.

[coveralls]

Coverage decreased (-0.1%) to 99.785% when pulling 5901fe9 on ottonemo:feature/s3-persister into 52d0aed on ottogroup:master.