Tidied up w.r.t. to leveraging the assets task.

The plugin wasn't fully part of the asset pipeline before and didn't write out its resources correctly. In addition files didn't appear to be written out relatively.

Writing to an incorrect location could cause issues with other pipeline plugins such as rjs.

Also did some general tidying up.

This plugin could do with some scripted tests - it is hard for me to determine if things have been damaged in the re-factoring.
This is a convenient approach for testing plugins. Simply reload the sbt console in the project and you'll get changes to the plugin without having to re-publish.