Multiple Framework Builds

[bangnoise]

Most people using Metal aren't going to want OpenGL, and vice versa.

I suggest:
One target, Syphon.framework, has the base classes
Another target, SyphonGL.framework, bundles Syphon.framework and adds the existing SyphonOpenGL classes
and then another target, SyphonMetal.framework, bundles Syphon.framework and adds SyphonMetal classes

[bangnoise]

@vade sound a good idea?

[bangnoise]

I'll do this after #88 and #89 to limit duplicated work once we have multiple framework targets

[danomatika]

Is there a new build with the Metal backend, ie. SDK 6?

I have integrated Syphon into a project, first as a submodule which builds the framework as a dependency however exporting an archive fails since Skip Install is set to No in the Syphon xcodeproj, resulting in a generic (non-app) archive. My backup is now to build a Syphon xcarchive via Makefile+xcodebuild, then copy the framework into my build tree for use by my main projects xcproject.

In either case, an SDK 6 build would be nicer. ;)