added banners for location with color

@ltw. Here's a screenshot of our progress with Pairing is Caring. http://i.imgur.com/QRTr51a.png

We implemented background colors and "enbiggenated" the font for mentor availability locations. Now hopefully people won't schedule sessions for cities they don't live in. Yaaaaay.
I'd like to see the colors defined as city-specific CSS classes rather than stored in Ruby. When I want to change the color of something, I to assume I need to update CSS not Ruby.