Shadow & Border Animation Support. New zPosition modifier behavior.

• prepare for tvOS release
• Add new zPosition modifier that animates zPosition from/to a given value. The old zPosition & zPositionIfMatched modifiers are removed. If you still want to modify the draw order, set the zPosition property on the view's layer before transition. Check the Apple HomePage Example for how to do this.
• Add shadow and border animation modifiers:
  • shadowPath
  • shadowRadius
  • shadowOpacity
  • shadowColor
  • shadowOffset
  • borderWidth
  • borderColor