Add an xcworkspace, move RAC reference into it

This allows an application project to use two different libraries with RAC dependencies.

 * When working on RCL directly, it should be built and tested from the `xcworkspace` file.
 * To add RCL to an application, set up a workspace for your app, and add the RAC and RCL **project** files to it. Then link RAC and RCL into your application target and Xcode will figure out the dependencies automatically.