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)
This will cause problems on Windows for people who build their own ensime-sbt plugin. The string literal will typically contain \r\n sequences. But msToSExp separates modules with \n alone. Also for some reason that I don't understand, a line that ends with ${...}  has a \n line ending. In any case, the generated .ensime contains inconsistent line ends. The best solution I can think of is this:
```scala
  def toSExp: String = s"""(
......
)""".replaceAll("\r\n", "\n")
```
to guarantee \n endings everywhere