Added ability for $controller to fetch controller constructor function when expression evaluates to string in scope

This PR adds support for the `$controller` service to evaluate an expression in scope and fetch the appropriate controller constructor function if the evaluated expression is a string.

This addresses [#3375](https://github.com/angular/angular.js/issues/3375).

Example usage:

    <div ng-controller="ExampleCtrl">
      <div ng-controller="currentRoute.controller">
        {{text}}
      </div>
    </div>

    function ExampleCtrl($scope) {
      $scope.currentRoute = {
        controller: 'FooCtrl'
      };
    }

    function FooCtrl($scope) {
      $scope.text = 'Hello World!';
    }