Snap to ways/vertices/midpoints when drawing (#240)

Also add anti-snapping behavior when option key is down.
Refactor operations

They're something distinct from actions. Actions are independent
of UI, operations are actions + UI (title, keybinding, modality,
etc.)
Move operation (fixes #536)
Remove unused DragWay behavior
better user icon style.
Merge branch 'master' of github.com:systemed/iD
Merge branch 'release' of github.com:systemed/iD into release
Fix Google Analytics include.