Change database to PostgreSQL

Remove all configuration for MySQL and configure PostgreSQL as the main
database.

All *_mysql_* options have been changed to *_db_* options.

Postgres requires the database user to have a password in order to
connect via localhost. The db_admin_password option is used to set the
password of the admin user (usually postgres).
Convert all varchars to text

PostgreSQL works similarly with varchar and text columns. Rather than
limiting the amount of characters a column can hold simply use text
columns.

Because this sql is now PostgreSQL specific, there's no need to maintain
what was set in the mysql settings.
Merge pull request #129 from ssoriche/postgres_only

Change database to PostgreSQL