Add PostgreSQL support to class.AjaXplorerUpgrader.php

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Change serial_role type to bytea for PostgreSQL (table ajxp_roles)

Output of php serialize() "is a binary string which may include null bytes,
and needs to be stored and handled as such". (see php manual)
We should store all serialization in BLOB/BYTEA column.

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>