Don't just check for the pdo extension, ensure pdo drivers as well
Label Postgres support as Experimental
Update install to focus on avaiable databases not drivers

I can't understand how users would care which extension they use to
connect to their database. These changes put the focus on which
databases are available to be used. The installer selects the best
available driver to use.

* The installer always chooses the PDO driver if one is available.
* If the selected database is MySQL the installer will choose mysqli
 over mysql