Eliminate duplicate getPreviousBuild functions

And move some stuff into CatalystUtils for use by other plugins.
Add a plugin for HipChat notification

This plugin sends notification of build failure or success to a
HipChat room, if the status differs from the last build.

The plugin can be configured by adding one or more of these stanzas to
hydra.conf:

  <hipchat>
    jobs = (patchelf|nixops):.*:.*
    room = 1234
    token = 39ab2198fe...
  </hipchat>

Here "jobs" is a regular expression against which the fully qualified
job name of the build is matched (so for instance
"nixops:master:tarball" will match the stanza above).