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.
It's worth making sure that what white text is going to be readable on all the product colours. Of the top of my head this should be ok.