Ensure that on EAGAIN we continue to try to spawn a worker.  When workers finish with a queue have them migrate to the other queues looking for work.
Merge remote-tracking branch 'origin/master' into gsb-workers-migrate

Conflicts:
	c_src/async_nif.h
Fix potential to use uninitialized value when branching.
Merge pull request #10 from basho-labs/gsb-workers-migrate

Worker threads should check for work in other queues before exiting.