rewrite of ensime-sbt

let's leave this open for a few days: I just want to raise awareness that the plugin is being rewritten to dramatically simplify it (it had grown up to work with older versions of sbt and had become confused)
at this point, only files are returned. To split javadoc/sources would involve two calls to updateClassifiers :-( I think ivy enforces the naming convention so we're ok with this hack.