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).
Merge in the first bits of the API work

The catalyst-action-rest branch from shlevy/hydra was an exploration of
using Catalyst::Action::REST to create a JSON API for hydra. This commit
merges in the best bits from that experiment, with the goal that further
API endpoints can be added incrementally.

In addition to migrating more endpoints, there is potential for
improvement in what's already been done:
* The web interface can be updated to use the same non-GET endpoints as
  the JSON interface (using x-tunneled-method) instead of having a
  separate endpoint
* The web rendering should use the $c->stash->{resource} data structure
  where applicable rather than putting the same data in two places in
  the stash
* Which columns to render for each endpoint is a completely debatable
  question
* Hydra::Component::ToJSON should turn has_many relations that have
  strings as their primary keys into objects instead of arrays

Fixes NixOS/hydra#98

Signed-off-by: Shea Levy <shea@shealevy.com>