Adds a sbt-sass plugin

I was in need of a sbt plugin for compiling Sass files at work. After some trial and error of testing the existing plugins out there I stumbled over this project and decided to port the sbt-less plugin.

I've included the necessary parts of the latest sass gem (3.2.14) and uses JRuby to call sass for compiling.

Could you please take a look and tell me what you think of it?

Regards 
Torbjørn