Enabling spatial support into model mommy

Hello @vandersonmota.

This is a great project, but in my day to day work uses a lot of geospatial information, so I need model_mommy to generate spatial data as well.

This a first draft that includes a point generator, using the already consolidated formulaes used by model_mommy.

In here I've updated the association between a field type and generators and created the spatial generator.

perhaps there are other ways to handle this, but let me know.