Rewrite variations for new colour system

This is my proposal for asimov-pill v3.0.0. A very simple change, modelled on the semantic variations in asimov-alert.

Here are the included variations: 
![screen shot 2014-08-22 at 10 36 01 am](https://cloud.githubusercontent.com/assets/37946/4005557/a96f7c30-2994-11e4-82ff-775074358c8f.png)

The previous 3 variations have been removed as the class names were direct references to the old colour system.

The `pill--tag` variation has been included to facilitate the use of tags we're now seeing across platform. Workbench and Contests are already using the asimov-pill mixin and setting the tag-specific background manually, this variation just cements that existing usage.

The `pill--primary` variation uses `colors/product/p0` as its background. It's one we need for Projects and I'd expect other products will benefit from having a pill available in their primary product colour.

Ping @xzyfer @joshamos for sanity check before release.