Exposed $event for use with ng-submit

Added support for accessing $event with ng-submit.

Example:

    <div ng-controller="submitCtrl">
        <form ng-submit="submitCallback($event)">
            <input type="submit">
        </form>
    </div>

    function submitCtrl($scope) {
        $scope.submitCallback = function(e) {
            console.log(e);
        };
    }