Also disable user-drag/prevent dragstart events

Unlike preventTextSelection, it appears important that
this happens on mousedown rather than the first mousemove
event. Otherwise, tile images are still sometimes draggable
on Firefox.
Remove preventDefault from BoxZoom too
Merge pull request #1786 from Leaflet/preventDefault

Remove preventDefault again, fix FF
minor formatting stuff
update changelog and build
fix regression of not being able to addLayer in initHooks
update build
add Circle & CircleMarker toGeoJSON
Use "wheel" event where supported (#1788)
Merge pull request #1789 from Leaflet/wheel

Use "wheel" event where supported (#1788)
bump version, update changelog and build
minor changelog fix
remove outdated changelog line
update docs section in contributing guide
bring back the horrible timeout hack for Android, close #1785
update build
merge master
finish 0.6 docs