Changed footer link 'Contributors' from text link in footer3 to graphic link in footer2, replacing Rails link

We moved the 'contributors' link from footer3 to footer2, where it now replaces the rubyonrails link. It uses an AgileVentures logo as an image link. Thanks Michael and Thomas for pairing on the earlier parts of this.

Staged at http://localsupport-northumbria.herokuapp.com/contributors

![av-contributors-link-in-footer2](https://f.cloud.github.com/assets/2813357/2447334/eb7d48d4-ae95-11e3-9a62-81ccde7182ec.png) Responsive View
![av-contributors-link-in-footer2-desktop](https://f.cloud.github.com/assets/2813357/2447374/4f8c69ea-ae96-11e3-8e4e-e9149c35b356.png)
 Desktop View