Use npmcdn.com instead of commiting build artifacts into git

[esamattis]

Keeping build artifacts in git is bad for so many reasons...

To do this we can just remove the dist from git and create it on npm prepublish hook.

Old school users can then just download the bundle from

https://npmcdn.com/underscore.string@3.2.2/dist/underscore.string.min.js

or even stick it to script tag directly.

[stoeffel]

SGTM