Update domain.py

timeline() now includes mentions
Users now have a simple profile that is displayed at the top of their @username page
	- A profile currently consists of a first name, last name, and personal message
	- fields can be easily added or removed, this is just proof of concept

	- User profile can be initialized when a user signs up
	- they can then be changed by clicking the "edit profile" link on the user's homepage

TODO's
	- It would be nice to extract some code from app.py, this doesn't seem like the place for some of this new logic.
	- Another nice feature would be a password editor (this has not been implemented because of the potential complications due to salting)
Added documentation for User class (WIP)