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
Updates the threadpool to use the Queue from threading
Merge pull request #505 from lukecampbell/threads

Updates the threadpool to use the Queue from threading
Filesystem changes
Merge pull request #506 from lukecampbell/fs

Filesystem changes
Ensures force_clean doesn't run for pycc tests

There are tests that inherit from PyonTestCase but are part of the INT group of
integration tests. The PyonTestCase adds a file system cleanup at the
conclusion of the test runs. This patch does a quick env variable check to make
sure that a file system clean is allowed.
Merge pull request #507 from lukecampbell/fs

Ensures force_clean doesn't run for pycc tests
Switch to gevent 1.0
Manually rebased test_async.py
Revert greenlet back to 0.4.0