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
Xcode upgrade added this. I think this is new behavior as retina Macs didn't exist when this project was made. It just combines any 1x and 2x artwork into one TIFF file to save space. Currently it has no affect on the build, since there's not artwork in the project.