Adding PostgreSQL support

Hello and thank you for making selfoss!  I noticed that the project doesn't presently support postgresql as a database, so I hacked up a patch to add that.  The versions I tested with were pg 9.0.7 and php 5.3.3-7 on debian stable i386; operationally I tested by importing my Google Reader OPML file and clicking around some.  The config.ini parameters are all the same, db_type would be set to pgsql (and port, likely, to 5432).  Probably any postgresql version 8.x or above would work (I checked the things I added against the 8.4 manual as I believe that is the version debian stable ships by default).

I'm guessing that anyone who wants to use this would know the relevant DBA tasks for creating a database and role, setting up authentication, etc.  There's a brief note in the header of the Database class pointing people at the online manual.