feat($parse): toggle evaluation of private fields via $parseProvider.allowPrivateFields()

Disallowing evaluation of private fields in Angular expressions should be toggleable through a `config()` block.  Usage is shown in ngdoc comments.

It's worth noting that this behavior defaults to FALSE.  

That is to say, if you want pre-Angular-1.2 behavior, you must do:

```javascript
myModule.config(function($parseProvider) {
  $parseProvider.allowPrivateFields(true);
});
```

Please let me know if the unit tests are insufficient; they do not consider multiple "formats" of private variables as the tests do in 3d6a89e8888b14ae5cb5640464e12b7811853c7e, but the main intent here was to show that the toggle works.