- Added new generic function "next_value()", accepts

- Added new generic function "next_value()", accepts
a Sequence object as its argument and renders the
appropriate "next value" generation string on the
target platform, if supported.  Also provides
".next_value()" method on Sequence itself.
[ticket:2085]
- added tests for all the conditions described
in [ticket:2085]
- postgresql dialect will exec/compile a Sequence
that has "optional=True".  the optional flag is now only
checked specifically in the context of a Table primary key
evaulation.
- func.next_value() or other SQL expression can
be embedded directly into an insert() construct,
and if implicit or explicit "returning" is used
in conjunction with a primary key column,
the newly generated value will be present in
result.inserted_primary_key. [ticket:2084]