Add can_consume_max_estimate() method to QoS object.

This method will be used by channels with bulk 'get'
capabilities to improve performance of get calls by
retrieving more than one message at a time.
Revert "Add 'messages_to_fetch' option to kombu.transport.SQS. Default to 10."

This reverts commit 8b3c74a9c3f415a1404a84446daa2c285632bba5.
Pep8 cleanup
Add test_get_with_empty_list method to check that Empty is raised
Pyflakes cleanup
More consistent test queue name
Use real Queue objects to test publishing/consuming messages in SQS tests.
Make _get() simpler, move most logic into private methods.

In preparation for building a _get_bulk() method to
handle getting many messages from SQS at once, this commit
moves most of the logic from _get() into separate methods
that can be more easily unit tested and re-used.