Change notes and components

In order to add Change Notes using the Document Footer in govuk-components to Specialist Frontend we needed to use the new grid_layout, which also allowed me to remove most of the SCSS dealing with inner and outer containers.

This PR also contains the work to turns Specialist Frontend into a mostly component powered App. The work to pass change notes in is a tiny part of this PR and included in a commit towards the end. 

Other changes included in this PR are as follows:

- bump Slimmer to get `core_layout`
- remove govspeak SCSS (as this now comes from components)
- use components for content
- remove unnecessary SCSS
- expose extra_date_metadata as a public method so we can pass it as `other` to the components
- change the footer to use a `document_footer`
- update cucumber steps to check JSON returned from slimmer

The tests need the following to happen before they pass: 
- [x] Merge https://github.com/alphagov/slimmer/pull/102 and bump it here

Page should look the exact same. Screenshots of the top and bottom:
![screen shot 2014-12-02 at 16 47 39](https://cloud.githubusercontent.com/assets/449004/5266655/fc108b1a-7a42-11e4-8aef-b7daead9c444.png)
![screen shot 2014-12-02 at 16 47 46](https://cloud.githubusercontent.com/assets/449004/5266656/fc1496c4-7a42-11e4-9dde-f12a907b094f.png)