Disable text selection on mousedown rather than move

This seems to fix #1800 and a related unfiled issue reported in
IRC regarding tiles becoming selected on double-click when a
vector layer is present.
Merge pull request #1801 from Leaflet/1800

Disable text selection on mousedown rather than move
fix chrome zoom anim flickering near map borders
update build and changelog
Replace explicit z-index with DOM reordering

Setting an explicit z-index interacts badly with tile
layers which have an opacity less than 1, and therefore
create a stacking context.

Fixes #1790.
Merge pull request #1804 from Leaflet/1790

Replace explicit z-index with DOM reordering
Workaround for FF user-select bug (fixes #1807)
update changelog
add plugin authoring guide, close #399
Manage happen.js with npm and upgrade it
Consistent mouse event propagation in Marker and Path

Marker was missing mouseup and mousemove.
Path was missing mouseup.
updates to plugin authoring guide
Merge pull request #1809 from Leaflet/mouse-events

Mouse events
Copyedit
update build, bump minor version
update changelog
bump version in package.json
rename plugin authoring guide
merge master