New banner for website

There was a big empty space of the previous banner of the website which I am replacing now with this one. I hope you like it.

On the side, I am reintroducing the compilation of the examples into html when we do in dipy/doc:
make html 
This option I think it was removed by @stefanv when he was fixing the API doc system. @stefanv let me know if this was intentional.