Add optional dependencies / exoskeleton support: first step

gh-754

This removes dependency on underscore methods to just three methods all of which are built into Exoskeleton. Which means you can now drop underscore del completely with Exoskeleton.

Removing `_.each` was a bit painful and made stuff *slightly* less readable, since I was not sure about min. supported IE version (by myself I use `Array::forEach` and `Object.keys` instead of `for`)