Use gevent.sleep instead of time.sleep

Use gevent.sleep instead of time.sleep to prevent blocking (just to educate the users not to use sleep):

http://stackoverflow.com/questions/12040880/how-to-avoid-blocking-code-in-python-with-gevent
Static analysis found some rough edges
Added travis tests for 2.5 and 3.1
Typo in travis setup script
Merge pull request #570 from FedericoCeratto/patch-3

Use gevent.sleep instead of time.sleep