Correct check for global `exports` for commonJS

(Also for browserify)

Using `typeof exports is not 'undefined'` in coffee script translates to
`(typeof exports === !'undefined')` which is obviously not what we want.
Instead we can use the expression `exports?` [1] which will properly
translate into `(typeof exports !== 'undefined' && exports !== null)`.

[1] http://stackoverflow.com/a/10238211/790169
Merge pull request #60 from EvanPurkhiser/fixup/correct-commonjs-check

Correct check for global `exports` for commonJS and Browserify