Geo: enchance Coord class

[zverok]

Possible implementations: GeoKit. Possible ideas:

• coord.distance_to(coord)
• coord.city (reverse geocoding)
• coord.route_to(coord) (routing)
• add more (look at Wolfram, please!)

Imporant consideration:

• everything for "just calculations" (without usage of external APIs) can go straight into Coord class
• everything with external API usage should go into separate (with possible opt-out in future) modules in extras/

[zverok]

More enchancements:

• Coord#latlng -- outputs string with lat,lng, ready to copy-past into map search
• Coord#links (?) -- constructs links to several popular cartographic services, like this page do

[zverok]

Still missing:

• .latlng
• may be .links?..

Out of scope of this ticket: route_to (will go to some "exras" eventually).

[zverok]

...also I've added sunset/sunrise, but no specs for them (and they always return UTC time, which is dumb).