Add Diffux logo to navbar

Included in this commit is the Diffux logo, graciously constructed by
malin.trotzig@gmail.com in 20 minutes from a draft published by
@lencioni in https://github.com/trotzig/diffux/issues/31. She made the
"UX" part capital letters and used the secondary color.

The SVG provided is the raw ouput from Adobe Illustrator. It might be
possible to optimize it, I could follow up with that.
Optimize diffux.svg by 13.1%

I used an online tool found here:
http://petercollingridge.appspot.com/svg-optimiser
Add "D" favicon

I cut out the "D" from the logo and put it in a 32x32 container with
some padding. Voila - a favicon.

Getting cross-browser support for a favicon is complicated, and I've
chosen a very pragmatic approach here, I'm not even caring by some
browsers.

Jonathan Neal has written a good post about the complexities:
http://www.jonathantneal.com/blog/understand-the-favicon/

I decided to go with a 32x32 pixel PNG put in a <link rel"icon">
tag. This standard way is supported by most major browsers (not IE10
though, but who cares (tm)).
Add original logo asset (diffux.ai)

This is the Adobe Illustrator file that malin.trotzig@gmail.com used
when creating the logo.
Add logo PNG asset (diffux.png)

This is the PNG that Adobe Illustrator produced when exporting the
diffux.ai to a 32 bit PNG file.
Remove rails.png

This image was added as part of initializing the Rails app. We're not
using it.