Expose original mouse event (+ other things)

[henryptung]

Make Leaflet.utfgrid events extend the original events

• This enables fetching other event-related metadata, like Ctrl/Shift modifier keys, off of the UtfGrid mouse event.
• Also did a drive-by fix of a non-primitive-on-class-prototype bug, which causes multiple UtfGrid instances to share the same request queue, causing some grids to lose tiles.
• Modified license comment to include @license annotation for better preservation by other dist-building tools. Wanted to have the uglifyjs version here do the same, but the current version is too old and didn't want to change the setup too much...
• Rebuilt dist from source updates? Not sure if this should be done now or at new version/release time.

[henryptung]

Hey, sorry for random unannounced PRs, but was using UtfGrid for a project and wanted to push some changes/fixes I made upstream back to the source.

Also, OOPS one of my commits is broken, will repush