Remove unused is_async
Rename future.wait to the more appropriate .join

This will also help to provide a common Greenlet/Future interface.
Add gevent-based async decorator
Test kill feature of gevent
Test gevent on Travis
Use concert.config to disable run-time features
Remove multiprocessing for needle computation
Move asynchronous tools to concert.async
Move coroutine things to concert.coroutines[.base]
Implement complete Future interface
Remove specific wait() functions

Ubuntu's stock gevent does not provide a module wait function, so we just join
all objects.
Provide gevent-specific threaded decorator
Force greenlet to start without .get() or .wait()