Add buttons styling (bug 931061)



Add the basic button styling. There's plenty more examples to add but this is a start. 

I've also moved style files around a bit and made more use of grunt to automatically import files from inc/ and lib/ so you don't have to keep importing the same old files.

As I'm pulling this across from webpay I'm refactoring the style so we'll have less style stuff to do when we get to the webpay SPA.

I'm making stylus files prefixed with '_' not be generated which is a fairly common pattern for partials.

*update*: Now uses flexbox (inc legacy flexbox support for b2g) which is better for not needing to worry about conditional padding of the content for the buttons to be at the bottom of the screen.

![buttons](https://f.cloud.github.com/assets/1514/1431729/a29c9e7e-40d3-11e3-8fab-abc25bf59c0c.png)
![two_buttons-2](https://f.cloud.github.com/assets/1514/1431733/af71dbaa-40d3-11e3-95e3-56087c732d1e.png)
![two_buttons-3](https://f.cloud.github.com/assets/1514/1431735/b4ef1048-40d3-11e3-890d-a42be5e97c6c.png)