Thanks to some tricks I learned from Nan Liu and Dan Bode, I was
able to figure out a way to move all of the new version-related stuff
back into the params class, and clean up some of the if/_real stuff.
Basic tests for centos6 + pg 9.2 are passing.
This commit creates a new class called `package_source_info`,
which has some initial framework for managing the postgresql.org
yumrepo. It also serves as a container for the 'version'
variable that is needed by the 'platform' class in order to
use other versions of postgres besides the system default.
