Add long-polling support to SQS transport

We've just released a [new version of boto](http://pypi.python.org/pypi/boto/2.7.0), which adds long
polling support for SQS.

#173 already has a discussion about adding long polling support
to the SQS transport, which is what I've implemented here.

The polling interval was set to 0 and a new
transport option (wait_time_seconds) was added.  This
parameter specifies how long to wait for a message from
SQS, and defaults to 20 seconds, which is the maximum
value currently allowed by SQS.

For testing, I verified that the tests in `funtests/tests/test_SQS.py`
still pass. I also verified that I see the appropriate API calls
being made to SQS as well as 20 seconds between the `get_messages`
calls.