Add __ne__ method to AudioSegment class

According to the reference (http://docs.python.org/2/reference/datamodel.html#object.__eq__),
when defining __eq__(), one should also define __ne__()
because the truth of x==y does not imply that x!=y is false.
Merge pull request #12 from clee704/add_ne_to_audio_segment

Add __ne__ method to AudioSegment class