Base post-receive hook now calls multiple scripts

It's a bit of a hack for now, it'll just find any post-receive-*.rb
scripts in the directory and load them all one by one. CONFIG is
loaded once from the yml at the start.
Remove config load from post-receive-hipchat

Config is now loaded once by the container so it shouldn't be loaded
by the scripts.
Update README after changes

Added information about installing for a single repo, and plans
for moving to a different config method.