feat(mdRadioGroup): Radio submits on keydown/enter

[marcysutton]

With this change, mdRadioGroup will submit a form on a press of the enter key. The directive checks for a parent node with the tagName of form and if it finds a match, it will trigger a submit event.

Closes #577

[googlebot]

CLAs look good, thanks!

[marcysutton]

@ThomasBurleson or @andrew I could use another set of eyes on this test....it's failing to spy on the form submit event even though the code works.

[ThomasBurleson]

I think a switch is better here:

switch(ev.keyCode)
{
  case $mdConstant.KEY_CODE.LEFT_ARROW:
  case $mdConstant.KEY_CODE.UP_ARROW:

    ev.preventDefault();
    rgCtrl.selectPrevious();
    break;

  case $mdConstant.KEY_CODE.RIGHT_ARROW:
  case $mdConstant.KEY_CODE.DOWN_ARROW:

    ev.preventDefault();
    rgCtrl.selectNext();
    break;

  case $mdConstant.KEY_CODE.ENTER:

    var form = angular.element($mdUtil.getClosest(element[0], 'form'));
    if (form.length > 0) {
      form.triggerHandler('submit');
    }
    break;
}

[marcysutton]

Closed with 03c7592