Answer login queries with a covering index

Creates a new index on auth_users that can answer login requests without
reading from the table. Minor optimization, but one that takes just a minute
to do and has few downsides, so might as well do it now.

Also makes username explicitly unique, since our login() funciton
assumes that fetchone() with a username will do the Right Thing.
Fix obvious syntax error: semicolons are a thing
Merge pull request #33 from jculpon/users-index-cover

Answer login queries with an index