Fix for cleaning up os calls through Popen

[gyglim]

This PR adds the same _del__ code for AudioFileClip as there is for VideoFileClip:
https://github.com/Zulko/moviepy/blob/master/moviepy/video/io/VideoFileClip.py#L104

Despite using moviepy master I had problems with readers not being closed (leading to OSError: [Errno 24] Too many open files), similar to:
#57
#255

I have a complicated setup with a server that runs a long time and is multi-threaded (and the issue happens after days). To find and fix the issue I wrote a wrapper around Popen to log the calls opened and closed:
https://gist.github.com/gyglim/340c9d276d7f53578eb9673156e556df

This Pull Request together with explicitly calling video.__del__() resolved the issue.
Explicitly calling __del__() relates to this:
http://stackoverflow.com/questions/1481488/what-is-the-del-method-how-to-call-it/1481512#1481512

When calling __del__() without the fix in this PR, the readers of AudioFileClip would stay open.

[coveralls]

Coverage increased (+0.02%) to 48.681% when pulling f612a45 on gyglim:fix_open_pipe into d633510 on Zulko:master.