Add the within_box method for querying against the bounding box only

We were working with big ranges (> 1500km) and noticed that computing the exact distance doesn't perform very well with a large dataset.
However, for such long distances, precision might not be an issue (at least it's not for us).
So this PR splits up the `within_radius` method in two separate scopes: one querying against the bounding box (`within_box`) and the other one computing the exact distance.
I've also written some tests.
Hope this fits to your lovely gem :dancers: