added group avatars

avatars can be added to groups and are shown on the dashboard and on the home of a project.
rspec and feature tests added

![screen shot 2014-01-27 at 20 54 52](https://f.cloud.github.com/assets/4224518/2013788/f03c26f0-879a-11e3-8db6-806d9b0987b3.png)
![screen shot 2014-01-27 at 20 54 58](https://f.cloud.github.com/assets/4224518/2013789/f1b34824-879a-11e3-9de2-c590aa388a5b.png)

avatars for projects are in a separate PR: #6161