Python 3 fix and documentation updates

• The description for org_by_name incorrectly directed people to use
  name_by_addr rather than name_by_name.
• Previously GeoIP.country_names was populated from GeoIP_country_name in
  the libGeoIP C API. Some versions of the libGeoIP include non-ASCII
  ISO-8859-1 characters in these names, causing encoding errors under Python
  1. When installed under Python 3, this API now uses UTF-8 country names to
     populate this dict. Reported and fixed by Sergey Farbotka. GitHub PR #15.