Merge pull request #221 from caroo/bugfix-dont-crash-return-nan

Avoid crashes during calculations. Geocoder.coordinates sometimes
returns nil which causes crashes in the distance and bearing calculations.
By returning a point array of [ NaN, NaN ] this distances will be also NaN
which seems to be a sensible result. If the argument to extract_coordinates
has an unexpected form, also return [ NaN, NaN ].