Attempt to get boundary geohashes to line up with PostGIS ST_GeoHash and geohash.org.
Reference: http://trac.osgeo.org/postgis/ticket/2201

Add asserts for hashes around boundary/pole conditions.
Merge pull request #8 from dhagberg/boundary-issue-branch

Attempt to get boundary geohashes to line up with PostGIS ST_GeoHash and geohash.org