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.
Merge pull request #88 from benbjohnson/tx-rename

Make DB/Tx API more consistent.