Makes models with Geoposition fields usable in admin inline forms

This attempts to fix the same issue as #6.

* Admin tried to save rows with models with Geoposition fields rendered as tabular inline even if they were not *dirty*, because the check to see whether the form had changed thought it had. This was caused by initialisation problems
* It also passes along **kwargs to the base field class, so that things like `initial` can get through