Add compiler plugin continuation to project settings
Changed implementation acording to Mikko Peltonen guides in the comment on the pull request.
(https://github.com/mpeltonen/sbt-idea/pull/203#issuecomment-13190607)

Extracted the scalacOptions mapping to a map and added translation of "-explanintypes", "-optimise" and "-nowarn".

The defined mappings will no longer be inclueded in the compiler options tag as idea don't do this.

All paths with -Xplugin: will be include in the plugin path list
Use the xplugin val to generate pluginPaths thanks to Eugene Platonov for pointing that out.
Add test for #203
Minor code cleanups
Merge branch 'issues/203' into sbt-0.12