Support Pointer events as well as MsPointer Events

Fixes #2037
I've taken a slightly different route to @DanielX2 in #2039. I did use your code for reference though, thank you very much :-)

Now the class is a generic Pointer -> Touch wrapper and it specifically listens to Ms prefixed ones if you are on IE10 where they exist (feature detected of course).

Please test, I've tested on IE10 touch and IE11 Desktop and it seems good.

Check this diff for the actual changes:
https://github.com/danzel/Leaflet/commit/bf3b660bef7c64bc85c1dd218f3e07f7133a1bff
Renaming messes up the diff viewer.