* Updated `smart_open` to accept an instance of boto.s3.key.Key as the
`uri` parameter.
* Refactored `S3OpenRead` and `S3OpenWrite` to require only a
boto.s3.key.Key instance to work, and get the bucket directly from its
properties. *This is not backwards compatible*.
* Commented out test `SmartOpenReadTest.test_s3_boto` temporarily.

Implemented @tmylk ’s recommendations:
* Fixed `test_s3_boto` by replacing usage of `lookup` with `get_key`
and re-enabled it;
* Fixed type checking in the constructors of `HdfsOpenRead` and
`HdfsOpenWrite`;
* Updated doctoring to reflect new usage of `smart_open`;
* Updated CHANGELOG to reflect this change.