add `watchify` dev utility and script for auto-rebuilding on file changes
add `watchify` dev utility and script for auto-rebuilding on file changes

PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs