Disambiguate un-cached shader locations from shader locations of zero
Add basic shader uniform setters
Update Makefile for OSX project to rebuild when core files are modified
Fix shader validity check doing the exact opposite of what it should
Automatically bind shader program when setting uniforms
Add comments to uniform setters
Change pointer usage to reference
Make ShaderProgram return values to const where appropriate
Clear cached shader locations after recompiling
If recompiling currently bound ShaderProgram, unbind it
Merge branch 'shader-uniforms'

Conflicts:
	Makefile
	core/tangram.cpp