Dialect Extension mechanism

* Moved `LIMIT ... OFFSET` to a limit-offset extension (used by both sqlite/pg)
* Moved `OR [REPLACE / ABORT / etc]` to sqlite extension
* Moved `RETURNING` & `DELETE ... USING` to postgres extension
* Removed `MINUS` support (Oracle only; if someone wants to create an Oracle extension and maintain it in their own repo, they're welcome to)
* Removed unnecessary layer of method-building functions

This pull request fixes #36.