Allow access to L.TileLayer imagedata

See https://developer.mozilla.org/en-US/docs/HTML/CORS_Enabled_Image. Seemed like such a small change it could maybe go in core instead of writing an entire plugin for it, but I also doubt it's a common use case. I use this to pull in data that's stored in tiles (e.g. elevation data, soil composition).
Merge pull request #2620 from erictheise/master

Update GridLayer.js
tile CORS headers now a layer option
removed trailing whitespace
Merge pull request #2621 from bennlich/patch-2

Allow access to L.TileLayer imagedata