Use edit-server package from MELPA.
Remove leftover exec-path assignment traces.

Originally done in 91e5c61.
Revert "Make feature-mode use rake binstub by default."

Binstubs are now stored in .bundle/binstubs, alleviating the need to
alter the path to the cucumber executable.

This reverts commit 92f6ea9b790faf76f24833514496b8861cd28ac7.
Add CoffeeScript mode setups.

- Flymake
- Override the mode's default(?) tab width to 2.