Add in pixel to rem, fix some minor requirements issues

[iamcarrico]

There were a few issues with the requires for testing, fixed those as well as added in a mixin to do pixel-to-rem, that will print out something like this:

@include pixel-to-rem(font-size, 18px);

font-size: 18px;
font-size: 1.125rem;

[Snugug]

Looks OK, but I need docs before I can merge it in

[Snugug]

Please reroll this against current head and only include the pixel to rem changes. Additionally, there are line notes that should be addressed.