Fix consistency in MessageRequest.poll

The help, example and actual code had different opinions on how the
function should behave. This commit make them agree.

poll() will return the base Request object until the receipt has
expired, been acknowledged or called_back upon. Once one of those three
things has occurred it returns None.

The example now has expire=120 and retry=60 since without those
arguments it is unable to send a message with priority=2.

The expired_at does not exist, it is called expires_at in the Pushover
API. This is fixed as well.

The values of expires_at, acknowledged_at and called_back_at can be
found as attributes in the MessageRequest object like before.