Implemented focus & better UIManager

- A widget can now be focused.
- The ingame menu now properly locks movement.
- UIManager is now fully polymorphic (no more coercions yay !).
All menus are now implemented
No more ugly option types in Game.ml

-Removed all option types in Game.ml
-Game is now a function (not a class)
Merging master
Merge pull request #86 from GL-MPRI-2014/gamemenus

Gamemenus
Improved Menus.ml (renaming)
Now with a confirmation popup !

-Added some classes, and a new module Windows
-Added a confirmation pop-up when forfeiting
-Added cool borders on menus and buttons
-All buttons/items are now activated by pressing Space
Merge pull request #87 from GL-MPRI-2014/gamemenus

Gamemenus
Trying to fix buttons...
Movement menu cannot be opened anywhere anymore.
Added .merlin to .gitignore, also updated README.
Documentation has been renamed...
Fix issue #84 on load screen
Added gui images
Using new icons in menus
Merge branch 'master' into generation
Merge branch 'generation' of https://github.com/GL-MPRI-2014/GL_MPRI_2014 into generation
Battlefield.to_string and create_from_string to compress the map into a (short) string
intended for server/client initial interaction