Changed how maps are loaded

All maps are loaded in the main game file. The current map is passed to
the level file. If you want to only load a specific map, just change the
levelIndex in PlatformerGame.cs. It's set to 0 right now, cause that's
my map. You each have a map.
Merge branch 'development' of https://github.com/Lakon/PiN into development
Merge pull request #10 from Lakon/development

Development
Merge pull request #3 from MikePaNtZ/development

Development