feat(ngForm): Supports expression in form names

<form name="ctrl.form"> form controller will accessible
as $scope.ctrl.form instead of $scope['ctrl.form']

BREAKING CHANGE:
If you have form names that will evaluate as an expression:

<form name="ctrl.form">

And if you are accessing the form from your controller:

  Before:

  function($scope) {
    $scope['ctrl.form'] // form controller instance
  }

  After:

  function($scope) {
    $scope.ctrl.form // form controller instance
  }

This makes it possible to access a form from a controller
using the new "controller as" syntax. Supporting the previous
behavior offers no benefit.