Added the ability to stream data using ``cp``.

This feature enables users to stream from stdin to s3 or from s3 to stdout.
Streaming large files is both multithreaded and uses multipart transfers.
The streaming feature is limited to single file ``cp`` commands.
Cut down memory usage to mutipart upload a stream.
Cleaned up the streaming code.

This includes adding more tests, simplifying the code, and some PEP8 cleaning.
Default s3 streaming to --only-show-errors
Updated changelog entry.
Merge pull request #903 from kyleknap/streams

Added the ability to stream data using ``cp``.