V2 theme refactor

Most notable changes:

- Deprecate color use cases
- Add anchors to demos for better testing
- Rewrite button generation pattern /cc @AlbertoElias there's less magic happening in there. Let me know what you think!

This PR also fixes many visual regressions introduced in the previous merge.

Idea for a next step: improve demos to cover more cases (elements, [backgrounds](http://jsbin.com/gezize/1/edit?html,output))
I've looked at the forms module, and it's specifying the font family:

https://github.com/Financial-Times/o-ft-forms/blob/master/src/scss/_mixins.scss#L10
https://github.com/Financial-Times/o-ft-forms/blob/master/src/scss/field-errors.scss
https://github.com/Financial-Times/o-ft-forms/blob/master/src/scss/prefix-suffix.scss#L49

For now I suggest to follow this convention. However I'll make it configurable.