Redesigned home & pricing pages

Building the new home & pricing pages and using this as a way to find out some basic requirements for the new splash page designs.

Including:
- New default btn styles (caps, heavier border - can use new `u-text-lowcase` to override)
- New spacing values (`xl` & `xxl` - no more nesting multiple `u-padding-Vl` etc to achieve now-frequently-used larger spacing)
- New colours
- /figures directory to keep separate the concerns of icon components & how they're modified in context (eg `.site-logo` used to set image background & dimensions, `.header-logo` used for header-specific rules to be applied to `.site-logo` component)
- Two header options (`_site_header.html` & `site_header_invert.html`)
- More!

Would like to investigate using SVGs for logos but will make separate pull request for this.

![the uk s 1 for online direct debit gocardless](https://cloud.githubusercontent.com/assets/883598/4303169/e6901588-3e62-11e4-9788-8bc7b5aae65a.png)

![pricing gocardless](https://cloud.githubusercontent.com/assets/883598/4303286/9557a206-3e64-11e4-9d5b-b7a10da52d94.png)