Add support for geohashes

[philippjfr]

Geohashes are a common way to represent parcels of land. It would be nice if we could support them as a data format, e.g. just like we have support for a column of shapely objects we could support a column of geohashes in a data interface which are then translated into a shapely polygon for rendering.

[jbednar]

See the examples in https://nbviewer.jupyter.org/github/Valassis-Digital-Media/theto/blob/master/theto_demo_notebook.ipynb for examples of using them with Bokeh as part of the theto project.