Added MySQLi support. Updated install process to allow choice, validation done.

Updated the db driver file to use new naming of db drivers (prepend with DB, i.e. DBMySQL, DBMySQLi as you cannot load the driver due to reserved object name (new MySQLi etc))
Fix for the default driver type.
Changed upgrade script to support enum database field type. Close #981
Merge branch 'v1.5.6.1' of git@github.com:opencart/opencart.git into v1.5.6.1