Fixes for the postgresql collector

The PostgreSQL collector is broken for running as a continuous collector
This patch also udpates it so that it does not make assumptions about
the column names

 - Map the metric names directly to the column names
 - Turn on autocommit on the connection, or the stats wont update
   , as pg_stat_database is only updated at the start of a
   transcation.
 - Fix a bug where only all databsae stats get mapped to the
   last database