Add stopAt(position), play() to Movie interface

Adds the following public methods to the Movie interface:

    /** Sets the stop frame for this Movie.
     *
     * @param position the int frame or String label to stop at.
     *
     * @return this movie for chaining
     */
    public function stopAt (position :Object) :Movie;

    /** Sets the movie playing.  Movie will automatically stop at its stopFrame, if one is set, 
     *  otherwise it will loop forever.
     *
     * @return this movie for chaining
     */
    public function play () :Movie;