Add @rpath support to SBJson.framework build settings

@rpath facilitates framework embedding in OSX applications:

1. Add 'SBJson.xcodeproj' as a subproject of **your** project
2. Go to **your** application target Build Settings and set 'Runpath Search Path' to `@loader_path/../Frameworks`
3. Add SBJson framework as a target dependency in application target Build Phases
4. Link SBJson framework in application target Build Phases
5. Copy SBJson framework to 'Frameworks' directory in application target Build Phases

NOTE: Should work the same with Xcode workspaces.