Refactored OSX App Delegates

* Scene start is now clearly within the ApplicationDidFinishLauching
* Removal of instance variables and using getter methods with memoization
* Size of the window is now clearly defined in a method
* Window is automatically centered on the screen
* Window automatically accepts mouse movement events
Merge pull request #24 from burtlo/osx-templates-refactor

Fix:
=> Refactored OSX App Delegates