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)
![screen shot 2014-01-27 at 20 55 11](https://f.cloud.github.com/assets/4224518/2013790/f34c0810-879a-11e3-9bd0-5d443188c7e9.png)
![screen shot 2014-01-27 at 20 55 15](https://f.cloud.github.com/assets/4224518/2013791/f5197290-879a-11e3-818e-e0057fb867fc.png)