First pass at 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
Another pass at dialog extensions
Fix parameterized things in FROM clause
Merge pull request #46 from Suor/dialect-extensions

Fix parameterized things in FROM clause
Removed sqlite file
Updated docs & readme
Merge pull request #43 from CSNW/dialect-extensions

Dialect Extension mechanism