Starting work on thread pool

- More Tests
- Successful AsyncResult for multithreaded code
Primitive thread-pool-ish thing
A functional thread pool
Comments and removes a print statement
Adds thread-exit synchronization

Big Note: If a timeout is placed on synchronizing the exiting of the pthreads,
and it is exceeded, the pool becomes inconsistent. I use a SystemError instead
of a timeout, to force a fast-fail.
Fixes comments and docstrings
Started work on the threading lib
Finishes porting _threading module, untested
Removes leading underscore in threading.py
Merge pull request #503 from lukecampbell/threads

Gevent and Threading, working together