Check PostgreSQL server version at runtime

Checks whether the server really runs a supported PostgreSQL version.  Fedora 19 is shipping PostgreSQL 9.2 and Debian 8 will be shipping with PostgreSQL 9.3 so it is increasingly likely that people who install the default PostgreSQL with their distribution may not be using a supported version.  Currently users get a cryptic message about the renamed procpid column if connecting to v9.3