adding avatar to projects

adding avatar to project settings page
added avatar removal
show project avatar on dashboard, projects page, project page
feature and rspec tests added

TODO:
see if the avatar must be shown on other places
design improvement?
new default no_project_avatar image

![screen shot 2014-01-25 at 19 53 16](https://f.cloud.github.com/assets/4224518/2002448/3bb7a128-85f6-11e3-84f1-27b148fe268c.png)
![screen shot 2014-01-25 at 20 00 23](https://f.cloud.github.com/assets/4224518/2002447/3bb794a8-85f6-11e3-92ef-aec1a6808684.png)
![screen shot 2014-01-25 at 19 53 29](https://f.cloud.github.com/assets/4224518/2002449/3bb80550-85f6-11e3-9e74-a8aae2c2130e.png)
![screen shot 2014-01-25 at 20 20 57](https://f.cloud.github.com/assets/4224518/2002450/3bb84ab0-85f6-11e3-922c-de00a88db82d.png)

EDIT:
I just added icons from the repositories. They can be of type png, jpg or gif.
These are overruled with an uploaded avatar.
And there is a separate default icon for when no custom one is available. (just as indicator; can be made nicer ;-)
Tests for the repo icon must still be added.
![screen shot 2014-01-26 at 21 15 50](https://f.cloud.github.com/assets/4224518/2004876/b3cb518e-86c6-11e3-8091-86fb6560e976.png)