Replace underscore with lodash.

This PR replaces Underscore with Lo-Dash.

Lo-Dash has [lots of features](http://lodash.com/#features) Underscore doesn't, and unlike Underscore, Lo-Dash is [tested in Node.js](https://travis-ci.org/lodash/lodash), follows semver, [lists its tested environments](http://lodash.com/#support), and has [plenty of automated tests](https://saucelabs.com/u/lodash).

Also, Lo-Dash [is now downloaded](http://npm-stat.vorba.ch/charts.html?package=lodash) more on npm [than Underscore](http://npm-stat.vorba.ch/charts.html?package=underscore) and has well over a thousand other npm packages depending on it.

I've also ran this through Ghost's unit tests to check it out.