HSApp example: add an app delegate

* Driving example for defining interfaces and class implementations
Add 'objc_interface' and 'objc_implementation'

* Free variables in the latter are not supported yet.
* We always generate a header file now.
Concept test: clean header file
Test HSApp: add app delegate
Implement Haskell variables referenced in objc_implementation
HSApp: Adding an NSTextView
HSApp: Call Haskell mkLine from ObjC method
Merge pull request #8 from mchakravarty/define-classes

Support the definition of interfaces and class implementations