Merge Vanilla and jQuery versions

The Vanilla JS version and the jQuery/Zepto version can now be used
through the same script. This makes it easier for people to use while
also increasing maintainability.

The README was updated alongside.
Allow for em-breakpoints

When using an object as the second parameter, the object keys can now
be in pixels, ems, or completely unitless. The object values can still
only be given in pixels.
Merge pull request #19 from RadLikeWhoa/master

Merge Vanilla and jQuery versions