v1.2.0

• Add transition.end. (#77)
• Add cancel events to transition.on.
• Improve the performance of transition.style and transition.attr. (5a939e1, #88, #87)
• Tween functions can now use this to refer to the current node.
• Fix removal of styles when the value passed to transition.style is null. (#89)
• Fix interpolation in transition.style and transition.attr in certain non-string cases. (#87)