v2.0.0

• [BREAKING] ResizeObserver now always expects a function as a child, to which
  it passes in a reference as an argument that can be added to the element to be
  observed.
• [BREAKING] Returning width and height as the second and third parameters
  instead of as part of an object
• [BREAKING] Now reports 1x1 size on before the observer is triggered instead of
  0x0
• Added tests using Karma