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)
After typing `Newsletter::Subscriptions` and the like a few times, `newsletter` made sense at that time - changing to `newsletter_subscriptions` to be more indicative :ok_hand: