feat($resource): expose promise instead of only $then

- Instance or collection have `$promise` property which is the initial promise.
- Add per-action `interceptor`, which has access to entire $http response object.

BREAKING CHANGE: resource instance does not have `$then` function anymore.

Before:

Resource.query().$then(callback);

After:

Resource.query().$promise.then(callback);

BREAKING CHANGE: instance methods return the promise rather than the instance itself.

Before:

resource.$save().chaining = true;

After:

resource.$save();
resourve.chaining = true;

BREAKING CHANGE: On success, promise is resolved with the resource instance rather than http
response object.

Use interceptor to access the http response object.

Before:

Resource.query().$then(function(response) {...});

After:

var Resource = $resource('/url', {}, {
  get: {
    method: 'get',
    interceptor: {
      response: function(response) {
        // expose response
        return response;
      }
    }
  }
});