/*
 * call-seq:
 *   play(mrl=opened_mrl, position=0, time=0) -> nil
 *
 * Start playing the stream. mrl can be omitted, if it was given using Stream#open
 * or a previous Stream#play. Position and time parameters are optional and cause
 * the stream to seek to given position (percent) or time (seconds).
 *
 * Play is executed in a separate thread. So the play status is sent as events 
 * (see Event, Stream#handle, Stream#wait).
 */
static VALUE xs_play(int argc, VALUE *argv, VALUE self) {