Poll keyword handler _keyword method returns None instead of regex pattern

If the poll keyword handler is registered as well as its subclasses, the poll handler can get picked up first when the message is routed to a handler. In the event that the base handler picks up the message, a NoneType error occurs. The bottom of that traceback is:

```
File "/home/rapidsms_raspberrypi/www/staging/env/local/lib/python2.7/site-packages/rapidsms/router/blocking.py", line 31, in receive_incoming
super(BlockingRouter, self).receive_incoming(msg)
File "/home/rapidsms_raspberrypi/www/staging/env/local/lib/python2.7/site-packages/rapidsms/router/base.py", line 199, in receive_incoming
handled = func(msg)
File "/home/rapidsms_raspberrypi/www/staging/env/local/lib/python2.7/site-packages/rapidsms/contrib/handlers/app.py", line 28, in handle
if handler.dispatch(self.router, msg):
File "/home/rapidsms_raspberrypi/www/staging/env/local/lib/python2.7/site-packages/rapidsms/contrib/handlers/handlers/keyword.py", line 55, in dispatch
match = keyword.match(msg.text)
AttributeError: 'NoneType' object has no attribute 'match'
```

Essentially, _keyword() in PollHandler returns None in the event that hasattr('keyword') is False.