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. In this case, it seems reasonable to make such implication hold by making `__ne__()` return the inverse of `__eq__()`.