Added foss4g2015 logo to front page

One drawback of our site layout is that its difficult to put stuff like this 'above the fold' anyway hopefully this will help...

![selection_021](https://cloud.githubusercontent.com/assets/178003/5001603/226e3d5a-6a00-11e4-9dcf-d822032f21b0.jpg)

# CC

@jmckenna
@rduivenvoorde