Add Sequel::Postgres::Adapter ALIAS

Need to see how the various Mysql extension are handled - perhaps this can be done a better way.
Use a shared cached for the in-memory connection so it can be shared nicely.
Add tests to deal with the Sequel adapters nicely.
Adding sequel gem as a development dependency.
Adding modifications to support Sequel adapters.
Remove some unnecessary comments.
Making SQLite work whether the connection returns results as
a hash or not.  Also cleaned up some unnecessary comments.
Merge pull request #22 from pnomolos/master

Add alias for Sequel::Postgres::Adapter to PG::Connection