Create a framework target for Mac project

These commits add a new target for the Ensembles Mac project to build an Ensembles framework. This makes it much easier to use Ensembles in a Cocoa application since one just needs to link against the framework and copy it to the `Framework` folder as a build phase.

It’s worth noting that the framework is using `@rpath` so the hosting application should add `@loader_path/../Frameworks` to its `Runpath Search Paths` for the framework to be correctly linked when the dynamic loader loads the executable. See https://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/DynamicLibraries/100-Articles/RunpathDependentLibraries.html for more info.