feat($compile): '=?' makes '=' binding optional

If you bind using '=' to a non-existant parent property, the compiler
will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception, which is right
because the model doesn't exist.

This enhancement allow to specify that a binding is optional so it
won't complain if the parent property is not defined. In order to mantain
backward compability, the new behaviour must be specified using '=?' instead
of '='. The local property will be undefined is these cases.

Closes #909
Closes #1435

test/ng/compileSpec.js

@@ -1857,6 +1857,9 @@ describe('$compile', function() {
             ref: '=',
             refAlias: '= ref',
             reference: '=',
+            optref: '=?',
+            optrefAlias: '=? optref',
+            optreference: '=?',
             expr: '&',
             exprAlias: '&expr'
           },
@@ -1993,6 +1996,33 @@ describe('$compile', function() {
     });
 
 
+    describe('optional object reference', function() {
+      it('should update local when origin changes', inject(function() {
+        compile('<div><span my-component optref="name">');
+        expect(componentScope.optRef).toBe(undefined);
+        expect(componentScope.optRefAlias).toBe(componentScope.optRef);
+
+        $rootScope.name = 'misko';
+        $rootScope.$apply();
+        expect(componentScope.optref).toBe($rootScope.name);
+        expect(componentScope.optrefAlias).toBe($rootScope.name);
+
+        $rootScope.name = {};
+        $rootScope.$apply();
+        expect(componentScope.optref).toBe($rootScope.name);
+        expect(componentScope.optrefAlias).toBe($rootScope.name);
+      }));
+
+      it('should not throw exception when reference does not exist', inject(function() {
+        compile('<div><span my-component>');
+
+        expect(componentScope.optref).toBe(undefined);
+        expect(componentScope.optrefAlias).toBe(undefined);
+        expect(componentScope.optreference).toBe(undefined);
+      }));
+    });
+
+
     describe('executable expression', function() {
       it('should allow expression execution with locals', inject(function() {
         compile('<div><span my-component expr="count = count + offset">');