New phase banner styles

* Remove state from phase-banner mixin as coloured background is no
longer used
* Update README to show new usage examples

Screenshots:

![desktop](https://cloud.githubusercontent.com/assets/417754/2956645/790c4a32-da93-11e3-9112-b0d3213a3aef.png)
![ipad retina](https://cloud.githubusercontent.com/assets/417754/2956647/7da3a932-da93-11e3-8362-228c7117054d.png)
![iphone retina](https://cloud.githubusercontent.com/assets/417754/2956649/83ec59ce-da93-11e3-8aa4-6df383cc1c4b.png)