adds workaround for Typeface class bug
adds activeLayers
deprecates addLayer as it was before
reinforces tilejson id check
adds pre-looper checks for current map layer in TileLoader
changes setTileSource to initialize provider instead of adding more queues (doesn't work)
substitutes downloaderProvider instead of adding a new one
uses MapView's getTileprovider in getLayer to reuse provider, resetting tiles
origin is no longer needed
Merge pull request #38 from mapbox/downloader-interrupt

Downloader interrupt