use 'coffeescript' package

[odedniv]

The coffee-script package is old and deprecated (didn't update for a while, doesn't allow new JavaScript syntax). The new package is coffeescript.

This pull request:

1. Tries loading coffeescript first and then coffee-script.
2. Gives a proper message when trying to use coffeescript extension without coffeescript loaded (was previously TypeError: Cannot read property 'compile' of undefined).
3. Changed the dev dependency.

I feel like coffeescript shouldn't be in devDependencies but in peerDependencies or something, but couldn't get it to work. Currently it simply cannot really fail loading coffeescript since it's a dependency.

Closes #126

[coveralls]

Coverage decreased (-2.5%) to 97.458% when pulling 1250667 on odedniv:coffeescript-package into 18c5d0e on jhnns:master.