Update README.md

Added an instruction regarding dependencies in the Examples section
fixed incorrect callback names in checking if callback is set
incorrect call to menu._setDrag when no handle is created
Merge pull request #49 from amitayd/readme_dependencies

Update README.md
Merge pull request #50 from amitayd/fix_callbacks

fixed incorrect callback names in checking if callback is set
Merge pull request #51 from amitayd/fix_menu_drag_error

incorrect call to menu._setDrag when no handle is created
Fixed callback events to fire on all menu icons that interact with canvas.