Ignore .sass-cache; #516

We're using sass for the redesign, but we don't need the sass-cache in
version control.
Redo the logo in Illustrator; #519

Now that we have more people working on Gittip we need the logo in a
standard format. Here it is in Illustrator. Note too that this is using
Edmondsans per @dmdj03's design and not Lato as previously.