Minor bug fixes

* When you initiate a producer with a non-existant queue, the queue is
  created. However this partition info is not reflected in <tt>KafkaClient()</tt>
  immediately. So, we wait for a second and try loading it again.

  Without this fix, if we do <tt>producer.send_messages()</tt> after creating a new
  queue, the library will throw a <tt>StopIteration</tt> exception.

* In <tt>SimpleConsumer()</tt>, the defaults are not as mentioned in the comments. Fix this (or do we change the documentation?)

* There was a problem with the way the consumer iterator worked.
  for eg: assume that there were 10 messages in the queue/topic
  and you iterate over it as -

  ``` 
  for msg in consumer:
       print (msg)
  ```

  At the end of this, <tt>offset</tt> that is saved is 10.
  So, if you run the above loop again, the last message (10) is repeated.

  This can be fixed by adjusting the offset counter before fetching the message

* Avoid some code repeat in <tt>consumer.commit()</tt>

* Fix a bug in <tt>send_offset_commit_request()</tt> invocation in <tt>consumer.py</tt>

* Fix missing imports