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)
Not escaping backslashes will interact negatively with ensime/ensime-server#549 , which interprets "\n", "\r" and "\t" as newline etc.  So on Windows, file names won't come out right.
Once you escape \'s, you might as well go all the way and escape double-quotes as well, like in the original code.