Allow listen_for to receive a timeout parameter

Example:

``` python
yield message.reply("Ok, I'll delete everything. Say `abort` in the next 10 seconds to stop me.")
heard = yield message.listen_for('abort', timeout=10)
if heard:
    yield message.reply('Phew... aborted!')
else:
    yield message.reply('Goodbye cruel world')
    yield bot.self.destruct()
```
