Fix issue 21 to make it works across foreign key relations

[scottBowles]

Here is some more work on #21, which I believe fixes the issue.

I tried to make this backward compatible, though I'm happy to have another set of eyes on that.

If we aren't worried about backward compatibility, I think it can be simplified further to the following:

# coding=utf-8
from django.db import models
from django.db.models import Func


class LlToEarth(Func):
    function = 'll_to_earth'


class EarthDistance(Func):
    function = 'earth_distance'


class EarthDistanceQuerySet(models.QuerySet):
    def in_distance(self, distance, fields, points, annotate="_ed_distance"):
        """Filter rows inside a circunference of radius distance `distance`

        :param distance: max distance to allow
        :param fields: `tuple` with the fields to filter (latitude, longitude)
        :param points: center of the circunference (latitude, longitude)
        :param annotate: name where the distance will be annotated

        """
        clone = self._clone()
        return clone.annotate(**{annotate: EarthDistance(LlToEarth(*fields), LlToEarth(*points))}).filter(
            **{'{0}__lte'.format(annotate): distance}
        )

Let me know if there are any issues!

[jneight]

Hello, I just updated the CI configuration in master branch, can you check if your PR is ok?

[scottBowles]

I merged the updated master into my branch and changed this PR's base branch to master so checks would run (previously I had #22 as the base). Hopefully they all succeed! Let me know if there's anything else I can do.