SpatialAssociations Plugin

As part of a location-aware project I've been working on, I've started
work on this SpatialAssociations plugin.

It provides model helpers like 'has_many' and 'belongs_to' for spatial
associations, those whose results are determined by the result of a
spatial function linking two models.

So far there's only one type, is_enclosed_by / encloses, but I hope to
include several more common spatial associations that would be
helpful.

I've just started up the github here: github.com/bspaulding/
spatialassociations.

I'd appreciate it if anyone who's interested would take a look and
tell me what you think of the idea, plus any associations you'd like
to see implemented.