initial implementation of the S3Storage

This implementation provides an S3Storage that can be used to store entities
attributes in an Amazon S3 storage.

It comes with an STKEY rql function to be able to retrieve the S3 object key
for the entity attribute (similar to BFSS's FSPATH rql function).

S3 connection configurations are attached to the S3Storage instance, like:

  s3_storage = S3Storage(s3_bucket)
  storages.set_attribute_storage(repo, 'File', 'data', s3_storage)