Stat Screen, Test Unit implemented

This pull request includes the following:

* A fancy stat screen is implemented. It shows the stats of the player, including level, current_hp and max_health.
* Right now, we can spawn the menu during the player turn by pressing [ENTER]. In the future, we would like to spawn it by right clicking. (not hard to do).
* A test unit, Monk, has been configured in main.js. 

More to come: More sprite animation sheets, more units!