A cleaner way to add _order field in dynamic inline admin

Amending ``fields`` as currently done interferes with modeltranslation fields magic and causes all translation fields on inline forms to be duplicated<sup>1</sup>.

We could add some special handling like in [b3a770](https://github.com/wrwrwr/mezzanine/commit/b3a7708d7b69ace3bf7c2c0f0a0b37e005b6adc4), but overriding ``get_fields()`` (and ``get_fieldsets()`` for Djangos < 1.7)  seems to be a cleaner solution.

[1] ``_meta.fields`` already contains translation field instances when ``__init__`` runs, while ``fields`` is assumed to only list original fields.