add server zooms
add server zooms

Defines an array of zoom levels for which tiles should be requested from
the server. Tiles for intermediate zooms are requested from the next
lower server zoom and resized accordingly. Currently, caching of
requests relies on the Browser and tiles are re-read, which is
suboptimal.

nrenner/leaflet-tilelayer-vector,ebrelsford/leaflet-tilelayer-vector,ebrelsford/leaflet-tilelayer-vector