Added parameter "default_dbname" to pdo_pgsql driver which can be used to override the default database

PostgreSQL allows the user to set the application_name is connecting
to database. It is useful for monitoring purposes.
Currently one could just manually add 'application_name=foo' to DSN,
but having a parameter eases setting it using DoctrineBundle.

We add the instancename parameter documentation for oci8 driver

add charset option for pdo_pgsql driver