Geomaps API: calculate distance in km (#681).

The Geomaps API has a method to calculate the distance between 2 geo coordinates in kilometer.

For high performance a flat-surface formula for a "Spherical Earth projected to a plane" is utilized.
See http://en.wikipedia.org/wiki/Geographical_distance#Flat-surface_formulae

Java API:
{{{
double getDistance(GeoCoordinate coord1, GeoCoordinate coord2)
}}}

REST API:
{{{
GET /geomap/distance?coord1=<lon>,<lat>&coord2=<lon>,<lat>
}}}
The distance is returned in the message body.

See #681.