Update reference related to touchZoom:center

( using the correct gh-pages-master branch now)
Documenting the accepted pull request https://github.com/Leaflet/Leaflet/pull/2527
Merge pull request #2530 from mtmail/patch-4

Update reference related to touchZoom:center
Updated reference for crossOrigin option
Merge pull request #2646 from bennlich/upstream-pages

Updated reference for crossOrigin option
#2501, wheelDebounceTime option documented