Fix $apply issues
Color occupied cells
Move MARS-related logic to Mars

The Mars class represents a MARS instance. This commit is the first
stage of refactoring that aims to separate MARS logic from display.
Separate display logic

The classes Display and Cell are responsible for providing information
to the view based on the state of MARS.
Display warriors in different colours
Remove references to old $scope
Display a yellow border around the active cell
Mark cells modified by a warrior + fix bomber
Set the core size to 100
Add README.md