A GeoHashGrid aggregation. Buckets GeoPoints into cells ...

...whose dimensions are determined by a choice of GeoHash resolution.

Added a long-based representation of GeoHashes to GeoHashUtils for faster evaluation in aggregations.
Created new GeoHashCellsAggregator based on LongTermsAggregator but which formats bucket results with GeoHash strings rather than longs.

See https://github.com/elasticsearch/dev/issues/59
Closes #59