Added a Button class. It's far from done.

So far we have:
 - Proper constructors, destructors, and assignment operators for this new Button class.
   - Although some constructors probably will be changed and added.
 - Internal members for position, image, and label (for the Button class).
   - No setters or getters yet, though.
 - Cmake set to compile Button.cpp
 - Documentation for everything done in the Button class thus far.