replace coffeescripted-sbt plugin (it was too slow)

it was too slow because it used Rhino.  so instead, use custom code that
installs the right version of the CoffeeScript compiler in the local
node_modules directory and then runs it as external process. way faster.

as a bonus, we get a newer CoffeeScript version (1.6.3 instead of 1.2.0)

fixes NetLogo/Tortoise #32
fixes NetLogo/Tortoise #33