Improved specs for avatar + added a warning whenever the uploaded image is not a square

- [x] improves the specs for avatars
- [x] add a warning whenever the uploaded image is not a square

![screen shot 2013-08-25 at 12 22 22 am](https://f.cloud.github.com/assets/362783/1021532/0cdbc144-0d0c-11e3-892b-2ab3f35ca0dd.png)