If alerts

We now have two types of alert. I've made them a component and will go around and update other alerts in apps to use the new ones. I know there were some issues with alerts in thorntree using hyperlinks and not working with a component so will look at that (still useful everywhere else).

I also removed a couple of old and pointless typography variables.

![image](https://cloud.githubusercontent.com/assets/814861/3808468/ebe8bccc-1c77-11e4-8f6c-d131a0dcc6f0.png)


... Good point. Also to be considered is whether we use the same `--subtle` convention we used with the [buttons](http://rizzo.lonelyplanet.com/styleguide/ui-components/buttons) (since the style is arguably the same):

![1407325625](https://cloud.githubusercontent.com/assets/708296/3826266/71666c04-1d5f-11e4-854a-be5de668f8b2.png)