Automatically add .coffee/.litcoffee extensions

While browserify may natively support extensions in the future, coffeeify can automatically support extensions by:

1. Finding **all** calls to `require("foo")`
2. Resolving the full path to see if it is CoffeeScript
3. Transforming to `require("foo.coffee")` if needed.  This allows browserify to pick up the proper dependency.

I wrote up some tests that should still be valid when native support is added :smiley:  As far as I've been able to gather, it looks like these are all of the issues related to getting native support into browserify:

https://github.com/substack/coffeeify/issues/1
https://github.com/substack/node-browserify/pull/336
https://github.com/substack/node-browserify/issues/326
https://github.com/substack/module-deps/pull/5
https://github.com/shtylman/node-browser-resolve/issues/19
https://github.com/shtylman/node-browser-resolve/issues/20
https://github.com/shtylman/node-browser-resolve/pull/1