Switch db layer to PDO

More testing is needed but I think that I have eliminated the crusty of
PEAR DB requirement. It turns out that there was mixed usage of it and
mysql function calls.

- replace PEAR DB with PDO
- start migrating settings into a single array to reduce global spam
- some cleanup of schema
- actually use the User data access object
- rename DataAccessLayer to Dao
- create AbstractDao to provide common DAO functionality