Newsletter signup

- Adds newsletter signup form at the contact section above the footer
- Updates contact section design
- Communicates with the public website (build F2614) via JSON to create newsletter subscriptions and expects 201 or 422 as response
- Displays subscription result at the top of the page using global alerts in the non-JS version
- Displays subscription result inline in the JS version
- Depends on:
   - [Public Website 1e66af1c](https://github.com/moneyadviceservice/public_website/commit/1e66af1c25e27e3daeaad4fe3a9c86632933dc88) for creating the actual subscriptions
  - [Dough PR #84](https://github.com/moneyadviceservice/dough/pull/84) to style the benefits list 
  - [Frontend PR #460](https://github.com/moneyadviceservice/frontend/pull/460) for displaying alerts

![screen shot 2014-06-30 at 11 02 16](https://cloud.githubusercontent.com/assets/966819/3428236/fdc4c46e-003d-11e4-85c9-f0a22640bccc.png)
Our design up until now has been to adopt a *tell, don’t ask* style.

The signature of the interactor should be **call { block } → obj** - if the optional code block is specified, then that will be run and its result returned.

Instead of *asking* `Core::Newsletter::Subscriber` for `success?` we should *tell* it what to do on failure by passing it a block.