Use analysis-core as a base

It seems like using the [`analysis-core`](https://wiki.jenkins-ci.org/display/JENKINS/Static+Code+Analysis+Plug-ins) plugin as a base for this one would be helpful. It would allow most of the code in this plugin to go away, could make it simpler to add things like `simplecov` support (cf. #11), and would provide richer trend graphing and other features.

That being said, I also feel like each of the metrics covered by this plugin should be split into their own plugins in that case, since there wouldn't be much common code if any left in this plugin.