Mobile-first approach to grids (and get rid of o-useragent)

The current implementation of grids has some great ideas but its main limitation is that it doesn't enable us to add layouts.

Adding a new layout size (for example: the [XS layout](https://github.com/Financial-Times/o-grid/issues/61)) would result in gaps where "no layout exists", that's where a mobile-first approach comes in to solve this problem makes design changes easier. 

### Files to review

- [src/sass/_variables.scss](https://github.com/Financial-Times/o-grid/blob/bf8e04af7da64707522054763296a5c4719ddc24/src/scss/_variables.scss)
- [src/sass/_main.scss](https://github.com/Financial-Times/o-grid/blob/bf8e04af7da64707522054763296a5c4719ddc24/src/scss/_main.scss)
- [src/js/getCurrentLayout.js](https://github.com/Financial-Times/o-grid/blob/bf8e04af7da64707522054763296a5c4719ddc24/src/js/getCurrentLayout.js): new JS helper to know what layout is currently displayed to the user

### Have a look
- [Test demo](http://kaelig.fr/ft/origami/o-grid/demos/test.html)
- [All demos](http://kaelig.fr/ft/origami/o-grid/demos/)

### Documentation
- [New README](https://github.com/Financial-Times/o-grid/blob/bf8e04af7da64707522054763296a5c4719ddc24/README.md)
- [SassDoc](http://kaelig.fr/ft/origami/o-grid/docs/sass/)

### Sass-mq

A Sass mixin that helps manipulating media queries in an elegant, mobile-first way. 

It mainly helps with breakpoint management and could also help with IE 8 Support ([read about how the Guardian uses it](http://www.theguardian.com/info/developer-blog/2013/oct/14/mobile-first-responsive-ie8)).

- [Documentation](https://github.com/sass-mq/sass-mq/tree/v3.1.0-beta.1)
- [SassDoc](http://kaelig.fr/ft/origami/o-grid/sass-mq/)

Let's have a chat about this very soon :) We'd like to ship it in early January and update this list of dependents as soon as possible: next-header o-ft-footer o-ft-forms o-ft-header o-hierarchical-nav o-techdocs.
large or extra large?