Changed the service to one chatroom

Ran into various problems* keeping track of users in multiple chatrooms
across multiple devices/tabs. Switching to a single chatroom in order
to allow for the most robust feature set possible.

*[mizzao:user-status has comprehensive server-side functionality, but
does not allow for the access of the connection._id client-side (which
would be required for removing the user from a specific chatroom upon
unload/close).]
Only subscribe to messages from timestamp of join

The client will only see messages entered after joining the chatroom.
Removed unnecessary collections

Chatrooms is no longer needed since this will only cover one chatroom.

ChatroomUsers is no longer needed because we will be covering online
statuses with mizzao:user-status & Meteor.users.
Merge pull request #2 from brendanfturner/change-to-one-room

Change to single chatroom service. Messages subscription now begins at timestamp of client joined.