Extract CoffeeScript into our first "interceptor"
Adds support for istanbul code coverage.

* For fear that glboal.$$cov... might be present w/o
istanbul, I wrapped the istanbul interceptor in a big
try catch

* To prevent the finding coffeescript.compile and
istanbul instrumentSync on every single file, I 
cache them.
Allow additional user-provided sourceTransformers
Adds configure for one-time global option setting.
Makes 'istanbul' support opt-in.

Presumably this will be appropriate for any future
built-in source transformers added to the project.
Merge pull request #26 from testdouble/master

Implements Istanbul coverage support and user-defined sourceTransformers