Improve model stringification methods

**Resolves issues #720 and #561.**

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- [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.
`__repr__` isn't even supposed to be defined there. Whoops.