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

Allow access to L.TileLayer imagedata
fix error dragging outside of window in FF, close #2610
fix error dragging outside of window in FF, close #2610
Merge branch 'master' of https://github.com/Leaflet/Leaflet

* 'master' of https://github.com/Leaflet/Leaflet:
  fix error dragging outside of window in FF, close #2610
Fix #2658 ZoomBox not disappearing on right click
Replace custom _onPreventDefault function
Merge pull request #2659 from Turistforeningen/fix-2658

BoxZoom is no longer affected by right click
Fixing URL template issue #2695
Merge pull request #2696 from cebence/tilelayerwms-urltemplate-bugfix

Fixing URL template issue #2695
add 0.7.3 changelog
upgrade deps
fix vector rendering regression
add and use L.DomUtil.empty, close #2600
fix rare error when dragging vectors, close #2706
remove element argument in requestAnimFrame
Set detectRetina's expected type as Boolean at code level to match documentation and usage
Merge pull request #2707 from Leaflet/detect-retina-boolean

Set detectRetina's expected type as Boolean at code level to match docum...