BF: Fixed memory leak in visual.MovieStim

Two problems caused a memory leak when creating multiple MovieStim
instances across the experiment:

1.)
The line "self._player._on_eos=self._onEos" creates a reference to the
MovieStim object that prevents it from being garbage collected, that is
__del__ was never called

Solution:
set _on_eos only on playback and reset it on movie pause, stop and eos

2.)
The __del__ function did not release the memory occupied by the movie.py

Solution:
replace self._clearTextures() (which is not defined in MovieStim) by
self._player.next() (which releases the memory)
Merge pull request #456 from frankpapenmeier/master

BF: Fixed memory leak in visual.MovieStim