* polishing of code and documentation
 * add few unittests; run them from the project home with:
   python -m unittest discover
 * changed default Nominatim instance to
   http://open.mapquestapi.com/nominatim/v1
   to avoid overloading of openstreetmap
   (s. http://wiki.openstreetmap.org/wiki/Nominatim_usage_policy )
 * add option osm_id plus osm_type to NominatimReverse
 * add zoom aliases (NominatimReverse)
 * add countrycodes selection
 * add url options to choose Nominatim instance
 * add NominatimException
 * factor out the HTTP request to NominatimRequest
 * fix lat==0 or lon==0 leading to no result
 * fix URL encoding