Make DB/Tx API more consistent.

I consolidated the DB.Tx() and DB.RWTx() calls into a single DB.Begin(writable bool) call. This is more consistent with the database/sql library.

I also changed the DB.Do() and DB.With() call to DB.Update() and DB.View(), respectively. This is more intuitive and more inline with other database verbiage.

Fixes #72.