simplified utils.gevent_* code
added test cases for Futures working with Greenlets (issue #13)
Added a netcall.concurrency subpackage

It defines a Future/Executor abstraction layer. There are implementations
for Gevent, Eventlet and Pebble (ThreadPool/Task). Greenhouse is TODO.
renamed test_concurrency_tools.py -> test_green_future.py
concurrency: extended ConcurrencyTools, refactored get_tools(), added base.FutureBase
ported the GreenRPC* classes to the netcall.concurrency API
get_tools() and get_zmq_classes() allow to force env=None
updated test runners
Merge remote-tracking branch 'origin/master' into concurrency_tools
tests/rpc_mixins: call service.start() from setUp()
added RPCCallsMixIn.test_timeout(), updated test runners
removed offending imports from netcall.threading.* (broken)
removed "YIELD_" from the special YIELD_* names, updated rpc_mixins
simplified generator code on server, updated tests
simplified the client generator code
added test_generator_cleanup
fixed concurrency.pebble.ThreadPoolExecutor
tweaked test runners
tweaked GreenRPCClient.call()
ported ThreadingRPC* classes to the concurrency API