Add mysql2spatial adapter; copied from mysql2 adapter
Revert "Simplify sort_by_distance_from and avoid adding accessors on objects"

Original implementation is necessary to add 'distance' field as
documented; can't just remove it!

This reverts commit 348a75732949316165884f2bd77d9be690c1901b.

Conflicts:
	lib/geokit-rails/core_extensions.rb