feat($state): make state data inheritance prototypical

Modify state data inheritance to use prototypical inhertiance rather than Angular's extend method to enable more flexibility in handling state data.

Author: chrislambe

src/state.js

@@ -41,7 +41,7 @@ function $StateProvider(   $urlRouterProvider,   $urlMatcherFactory) {
     // inherit 'data' from parent and override by own values (if any)
     data: function(state) {
       if (state.parent && state.parent.data) {
-        state.data = state.self.data = extend({}, state.parent.data, state.data);
+        state.data = state.self.data = inherit(state.parent.data, state.data);
       }
       return state.data;
     },

test/stateSpec.js

@@ -1338,6 +1338,7 @@ describe('state', function () {
       expect($state.current.name).toEqual('HHH');
       expect($state.current.data.propA).toEqual(HHH.data.propA);
       expect($state.current.data.propB).toEqual(H.data.propB);
+      expect($state.current.data.hasOwnProperty('propB')).toBe(false);
       expect($state.current.data.propB).toEqual(HH.data.propB);
       expect($state.current.data.propC).toEqual(HHH.data.propC);
     }));