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.
Merge branch 'huntc-tidy-up-wrt-part-of-assets'
- added sbt-digest and sbt-rjs plugins to example project
- removed --sourcemap parameter from sass command line. it prevents deprecation warning. fixed #12
- compass parameter turned back in play-sass-example