Add praw-multiprocess tool and associated Multiprocess handler.

This commit provides the initial version of the Multiprocess handler. The
executable `praw-multiprocess` isn't likely to change other than adding some
command line argument parsing conveniences. The handler itself requires some
work. Notably:

* The rate limiter needs to be made thread safe.
* Caching should be employed (and threadsafe).
* The exceptions returned through the socket need to be raised.

Additionally, some sort of exception handling behavior should be defined for
instances where the praw-multiprocess program crashes, or is not available.