More Coffeescript options, altered Closure Compiler options:

  - Added `JsKeys.coffeeVersion` setting to switch back and forth between Coffeescript 1.1.0 and 1.6.1;

  - Added `JsKeys.coffeeBare` flag to toggle top-level function wrappers around Coffeescript files;

  - Renamed `JsKeys.optimisationLevel` to `JsKeys.compilationLevel` to be more in line with Google's nomenclature;

  - Altered the types of `JsKeys.warningLevel` and `JsKeys.compilationLevel` to introduce meaningful values,
    and introduced enumerations of possible values to make them easier to select.

  - Updated the `sbt-js` README to fix typos and document new features.