singletons rewritten + private members introduced

fixed flat buildings from rendering tall buildings too
data cache created
min allowed zoom level decreased to 15
Merge branch 'loop' of https://github.com/kekscom/osmbuildings into loop

Conflicts:
	dist/L.BuildingsLayer-debug.js
	dist/L.BuildingsLayer.js
	dist/OpenLayers.Layer.Buildings-debug.js
	dist/OpenLayers.Layer.Buildings.js
	src/Data.js
	src/Shadows.js
	src/render.js