Use reply_to instead of room, when possible

The property `reply_to` on the received message should be used as the room handle when calling `robot.messageRoom`. Previously, `room` was used, which only contains the name of the room.

When using the hipchat adapter, these may differ a lot. When `room` is, e.g., `Foo room`, the `reply_to` property may be something like `114223_foo_room@conf.hipchat.com`.

This was reported by Nick Jüttner - thanks.