Spawn the commit thread only if necessary

If there are no messages being consumed, the timer keeps
creating new threads at the specified intervals. This may
not be necessary. We can control this behaviour such that
the timer thread is started only when a message is consumed
Optimize auto-commit thread

The previous commit optimized the commit thread such that the timer
started only when there were messages to be consumed. This commit
goes a step further and ensures the following:
* Only one timer thread is created
* The main app does not block on exit (waiting for timer thread to finish)

This is ensured by having a single thread blocking on an event and
keeps calling a function. We use events instead of time.sleep() so
as to prevent the python interpreter from running every 50ms checking
if the timer has expired (logic copied from threading.Timer)
Fix an issue with thread argument
Merge branch 'master' into lazythread
Merge pull request #31 from mahendra/lazythread

Optimize auto-commit process
Merge branch 'master' into partition

Conflicts:
	kafka/consumer.py