UI crosshairs & pause menu

How to use?
Import the package into the game and place the UI prefab in your game.
Check if EventSystem is automatically created in your game, of not add
it yourself with create -> UI -> EventSystem.
Afterwards the imported UI prefab can be deleted.
Also remove the (old) crosshair script from the _GameController prefab.

What’s new?
The previous crosshairs used the old Unity GUI system and a (partially)
script from internet. These new crosshairs use the new Unity 4.6 UI
elements. Which also makes the code copied from internet redundant, so
those lines of code are removed.
Also a basic pause menu screen has been set up. Currently only controls
for customising the crosshairs are implemented. But it’s very easy to
add more stuff later on.

Note: opening the pause menu doesn’t actually pause the game yet
Removed reference to crosshair code

Since we don’t use any of the code copied from internet for the
crosshairs anymore. The reference can be removed.