Improve model stringification methods

**Resolves issues #720 and #561.**

---

- [x] Rename `display_fields` to `sort_fields`.
- [x] Make `__unicode__` and `__str__` return roughly the same thing (modulo encoding issues).
- [x] Make `__repr__` return something of the form `<Model: info>`, as recommended by the Python docs.
You missed a `{}` here, which leads to the system name not being shown.
```
>>> s = System.objects.all()[0]
>>> s
<System: >
>>> s.name
u'kec1108a-ap7801-r11-2'
```