Transparent Skin

This is a new version of #43. See that old PR for more details.

What's new in this version?

1. The old default skin is still available by using `data-montage-skin="wireframe"`.
2. The icons for Checkbox and RadioButton are now inline SVG. -> No extra request, fixes the loading delay when pressed, let's you easily change icon color with `.digit-Checkbox-icon { fill: green; }`.

Overview of the 4 skins:

![digit-transparent](https://f.cloud.github.com/assets/378023/1875347/1f0ab2e8-78f1-11e3-9d3b-05f09bb323db.png)

> Note: The blue is just an example. If you don't use any `color` at all, it would default to `black`.