Added targets to Xcode project to build gameplay.framework

I added targets to gameplay's Xcode project to build the product as framework. This will not affect any projects using gameplay as current configuration.

To build gameplay as framework, open the gameplay.xcodeproj and select target framework-ios or framework-macosx then build. The product directory will contains gameplay.framework which can distribute easier than multi-static libraries.

Users of gameplay can add gameplay.framework to there project and do not need to care about search path for include and libs.