[#1808] detect SuSE postgres a bit better

Also, some better documentation of the different operating systems
that are understood and detected in the postinstall.sh script.

Now explicitly covered are:

SuSE 11 and 12
CentOS 5 and 6
Fedora 17
Ubuntu 10, 12, and 13

across both postgresql 8.x and 9.x

This should close #1808 as reported by @freakrobot.