Added Gravity and basic Collision modules. Next steps: work on improving collision modules (right now they only detect collisions).
Sphere colliders will now move objects out of the way.
Finished collision between squares and circles
Merge branch 'physics' of https://github.com/codeLogik/XNA_GameEngine_CS680.git into pre-release

Conflicts:
	XNA_GameEngine/XNA_GameEngine.suo
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/CoreMain.cs
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/Debug/DebugPlayerObject.cs
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/Debug/DebugTools.cs
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/XNA_GameEngine.csproj.Debug.cachefile
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/bin/x86/Debug/XNA_GameEngine.exe
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/bin/x86/Debug/XNA_GameEngine.pdb
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/obj/x86/Debug/DesignTimeResolveAssemblyReferencesInput.cache
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/obj/x86/Debug/XNA_GameEngine.csproj.FileListAbsolute.txt
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/obj/x86/Debug/XNA_GameEngine.exe
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/obj/x86/Debug/XNA_GameEngine.pdb
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngine/obj/x86/Debug/cachefile-{ACA51E83-0369-43BB-88E7-B9ED035CD0B6}-targetpath.txt
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngineContent/XNA_GameEngineContent.contentproj
	XNA_GameEngine/XNA_GameEngine/XNA_GameEngineContent/obj/x86/Debug/XNA_GameEngineContent.contentproj.FileListAbsolute.txt

Merged physics into pre-release branch and fixed all conflicts, including binary files.  May need to fix up each local copy of the visual studio project files if they misbehave.