Gevent integration

Here we go with a PR for #159.

All tests pass and killing works for me. As said in #159, gevent will only work for Python 2. Some notable changes:

- `Future.wait` is now `Future.join`
- Changing the run-time configuration is done in `concert.config`. Before it was kind of a race condition in `concert.helpers`.

Some missing pieces:

- [ ] The `GreenletFuture` does not yet implement the whole `Future` interface
- [ ] Greenlets spit out error tracebacks on stdout. So often it looks like there is a problem with the tests but in fact, there is none.
- [ ] We need to find a way to test gevent support on Travis.
- [ ] @sensej: You will hate me for this but I think we should have an async module containing the (who would've thought) async stuff and move the coroutine basics into `concert.coroutines`. `Bunch` and `Command` could stay in the helpers module.