photutils logo

I created a logo for photutils:

![photutils_banner](https://cloud.githubusercontent.com/assets/4992897/5150179/07ae1ff8-719e-11e4-92b0-7e3e7c1473f3.png)

This PR adds the logo to the docs and customizes the HTML theme.  I'm currently working on more doc additions and improvements before the `0.1` release....