replace moodlelogo gif with png

The moodlelogo.gif in the footer is pretty jagged when it's not on a white background.  Here's a png version that's a smaller file size.
![screen shot 2014-02-27 at 10 29 27 am](https://f.cloud.github.com/assets/743499/2278596/79c4c77c-9f58-11e3-8d9b-7354f037fdc0.png)