Make ado adapter handle disconnecting an already disconnected connection

One of the integration specs is that this operation should not
raise an error, but it does in the ado adapter.  This just swallows
the specific error raised.
Emulate offsets on Microsoft Access using reverse orders and total counts (Fixes #661)

This is terrible for performance, but I think that's better than
not working at all.  It's still has some corner cases, such as
requiring that you select the columns you are ordering on, but
hopefully they won't be problematic.

Note that this should work for Microsoft SQL Server 2000 as well,
though we don't support that.  For people using Sequel with
Microsoft SQL Server 2000, they can manually extend their datasets
with the offset emulation module.
Deprecate JSON.create_id usage in the json_serializer plugin

JSON.create_id is a relic from when the json library automatically
create model objects, and should not be used at all any more.
Fix bad example code in xml_serializer plugin
Add Sequel.object_to_json to allow the use of alternative JSON implementations (Fixes #662)

Sequel already has a Sequel.parse_json method that all JSON parsing
goes through, this adds a method that all JSON serializing goes
through.  This should allow usage of an alternative json library
simply by overriding these two methods.

Note that if you change the json library Sequel uses, you may need
to make it handle Sequel::Plugins::JsonSerializer::Literal objects
correctly if you are using to_json with associations in the
json_serializer plugin.