Allow specification of message type on chat obbjects for improved display in chat window.

- Adds a msg_type property to chat objects with a default type of
  'message'.
- Sets the new msg_type property to 'notice' for things like user's
  joining/leaving a room and song changes.
Change chatroom style to distinguish between 'notice' and 'message' chat objects.

- Chat objects with a msg_type of 'notice' are grayed out, and fade into the
  background a bit more so that it's easier to scan the chat room and ignore
  'notice' type messages.
Merge pull request #2 from eojthebrave/eojthebrave-chat_message_types

Improve display of notice vs. message content in chatroom