rtree lookups and joins

[alimanfoo]

Proposed to add functions based on bounding box queries via the rtree module - http://toblerity.github.com/rtree/tutorial.html

E.g., bboxlookup(), bboxlookupone(), bboxjoin()

[alimanfoo]

Probably also bboxrecordlookup, bboxrecordlookupone, facetbboxlookup, facetbboxlookupone, facetbboxrecordlookup, facetbboxrecordlookupone, bboxleftjoin, bboxnearestjoin.

[alimanfoo]

Also, bboxaggregate, bboxcounts.