Fix the migration's table field description for mysql driver

Type was CHAR(255) were it should have been VARCHAR(255). Not that a
performance gain or loss is expected, but consistency and good practices
are to be followed as much as possible.
Added the postgresql driver

Added the psotgresql driver based on the work done by cjhubert on its
pull-request.
Merge branch 'cjhubert-postgres-driver'