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

![profile_-_zogstrip_-_discourse](https://f.cloud.github.com/assets/362783/1021415/d12b6e62-0cff-11e3-90a9-6f208f9626f6.png)