Removing unnecessary sleeps.

SQS is fast enough to handle these tests without it.
Removing gevent monkey patch from jobs.
Adding Queue.num_jobs method.

This tells us how many jobs are in the queue currently (as an approximation).
Converting Queue.jobs into a generator.

This will continue iterating over existing jobs until exhausted.  It still grabs
jobs in the cheapest and most efficient way.
Removing monkey patch.
Simplifying worker code.
Making num_jobs a method.

I figured it would be slightly misleading to leave it as a property, since
properties usually don't correlate to API calls, which this does.
Refactoring queue / worker tests.