Clever logo

- Fixes a bug whereby when CSS is disabled the user would see two logos
  side-by-side. The sprite now includes a black logo, and white logo
  which will remain hidden on a white background when CSS is disabled.

- Removes the need to include a solid green background as the black logo
  will now display when CSS is disabled.
Merge pull request #563 from moneyadviceservice/clever-logo

Clever logo