New API for checking pending message count
New API for getting a specified set of messages

This will be easier to use in some cases where we have to get
only a specified set of messages. This API uses the __iter__
API internally, but maintains the state to give back only the
required set of messages

API is - get_messages(count=1)
Missed a doc string
Auto commit timer is not periodic

The auto commit timer is one-shot. After the first commit, it does
not fire again. This ticket fixes the issue.

Also, in util.ReentrantTimer(), some duplicate code was cleaned up
Merge pull request #23 from mahendra/autocommit

Auto commit timer is not periodic
Adding a debug statement
Closes #22

Removed get_messages API, added test for get_pending
Merge branch 'issue-22'

Conflicts:
	kafka/consumer.py