Added a new argument to :class:`.Enum` and its base

Added a new argument to :class:`.Enum` and its base
:class:`.SchemaType` ``inherit_schema``.  When set to ``True``,
the type will set its ``schema`` attribute of that of the
:class:`.Table` to which it is associated.  This also occurs
during a :meth:`.Table.tometadata` operation; the :class:`.SchemaType`
is now copied in all cases when :meth:`.Table.tometadata` happens,
and if ``inherit_schema=True``, the type will take on the new
schema name passed to the method.   The ``schema`` is important
when used with the Postgresql backend, as the type results in
a ``CREATE TYPE`` statement. [ticket:2657]