Update Game_mac build for Xcode 6

Here are some improvements to xcode project in case people need to modify it

* Updates Xcode build settings to allow Game.app to be built on Xcode 6+
* Adds sandbox support to allow games to be submitted to Mac App Store
* Removes build folder that doesn't need to be in source control

Remaining issues:

* App crashes in full screen due to OS X 10.9+ changes. I think this needs to be fixed in OgreOSXCocoaWindow.mm
* App segfaults if closed using window's red X button instead of through game menu. Not sure how to fix yet
Worth making a note of?