_createTileProto removed after v0.6.2
update overzoom dependency
extracted web worker to separate class
AbstractWorker as base class + dummy
hide tiles when layer is added
progress layer improvements
hide by default, show on tileloading event
hide aborted tiles
fix tile error and abort handling
simple tile cache for overzoom
don't add aborted tiles to cache
remove try/catch
update Leaflet to v0.6.4
Merge branch 'dev' into gh-pages