Remove unnecessary trailing semi-colons
Define __ne__ to fix inequality comparisons between futures

This fixes the following bug:

   In [1]: from django_date_extensions.fields import ApproximateDate

   In [2]: ApproximateDate(future=True) != ApproximateDate(future=True)
   Out[2]: True

Previously, only __eq__ was defined for ApproximateDate, but
the Python documentation says:

  "There are no implied relationships among the comparison operators.
   The truth of x==y does not imply that x!=y is false."

  http://docs.python.org/2/reference/datamodel.html#object.__eq__

So, introduce __ne__ for ApproximateDate with tests for this
behaviour.