Added comment.
Added step sessions.

Allows mapper invocations to stash data for its complimentary combiner and reducer invocations.

Also:

- Moved handler sourcecode-synchronization logic from script to mr.handlers.sync .
- Moved handler execution to actual language-specific processor.
- Upgrded *etcd* version requirement.
- Added script to view sessions under a particular request.
- Bugfixes.
- Upgraded Gunicorn version requirement, to rid ourselves of annoying log verbosity.