Add spherical projection for globe-like view

[krisalyssa]

Google Maps switches to a spherical-ish projection when you zoom out far enough. Can we do the same?

I think Leaflet supports non-rectilinear projections, but some investigation is needed. A complication may be that since our source data (the tiles extracted from KSP) are based on a rectilinear projection, they may not look so good if converted to a spherical projection.