Initial import of GeventConnection and cleanup

This is currently NOT working and incomplete.
Fix docstring

Should be 'greenlet' not 'GreenThread', since it works on any greenlet type.
Reformat docstring
Add TEvent class that is based on gevent's Event

This is API-compatible with threading.event (hence the name) and has been added
to facilitate porting gevent support modules.

Closes #11
Clean up code
Fix error checking; reference guv.exceptions
Fix import error, import from source module
Add patcher support for cassandra
Rewrite green threading modules
Add threads example; revert back to original green thread modules
Fix TEvent to use guv modules correctly
Add calls to gyield() to ensure that we're in the same OS thread
Fix module-level attributes

For some reason, inject() isn't doing what I want it to do. Need to revisit this
later.
Add check to ensure that we're in the same OS thread
Patch threading.Event
Remove debug print statement
Show current thread
Add cassandra example
Replace select() with trampoline()

We shouldn't ever need to use select(). trampoline() is almost always what we
want when we're "blocking" for I/O.