feat($compile): add support for $observer deregistration

In order to make the behavior compatible with $rootScope.$watch and $rootScope.$on methods, and
make it possible to deregister an attribute observer, Attributes.$observe method now returns a
deregistration function instead of the observer itself.

BREAKING CHANGE: calling attr.$observe no longer returns the observer function, but a
    deregistration function instead.

    To migrate the code follow the example below:

    Before:

```
    directive('directiveName', function() {
      return {
        link: function(scope, elm, attr) {
          var observer = attr.$observe('someAttr', function(value) {
            console.log(value);
          });
        }
      };
    });
```

    After:

```
    directive('directiveName', function() {
      return {
        link: function(scope, elm, attr) {
          var observer = function(value) {
            console.log(value);
          };

          attr.$observe('someAttr', observer);
        }
      };
    });
```

Closes #5609

test/ng/compileSpec.js

@@ -1914,15 +1914,14 @@ describe('$compile', function() {
 
 
   describe('interpolation', function() {
-    var observeSpy, directiveAttrs;
+    var observeSpy, directiveAttrs, deregisterObserver;
 
     beforeEach(module(function() {
       directive('observer', function() {
         return function(scope, elm, attr) {
           directiveAttrs = attr;
           observeSpy = jasmine.createSpy('$observe attr');
-
-          expect(attr.$observe('someAttr', observeSpy)).toBe(observeSpy);
+          deregisterObserver = attr.$observe('someAttr', observeSpy);
         };
       });
       directive('replaceSomeAttr', valueFn({
@@ -2020,6 +2019,18 @@ describe('$compile', function() {
     }));
 
 
+    it('should return a deregistration function while observing an attribute', inject(function($rootScope, $compile) {
+      $compile('<div some-attr="{{value}}" observer></div>')($rootScope);
+
+      $rootScope.$apply('value = "first-value"');
+      expect(observeSpy).toHaveBeenCalledWith('first-value');
+
+      deregisterObserver();
+      $rootScope.$apply('value = "new-value"');
+      expect(observeSpy).not.toHaveBeenCalledWith('new-value');
+    }));
+
+
     it('should set interpolated attrs to initial interpolation value', inject(function($rootScope, $compile) {
       $rootScope.whatever = 'test value';
       $compile('<div some-attr="{{whatever}}" observer></div>')($rootScope);