add didChangeItemAction method and bug fixed

• add didChangeItemAction method (item is changed(scrolled))
• Bug fix : ‘updateSwitcherAnimation’ method execute repeatedly after view controller is dismiss at previous version(1.0.1) .
  In this version(1.0.2), fix the critical bug.

1.0.1

• Fix access modifier bug.(Access modifier was not properly set in 1.0.0 version.)
• Change demo gif files.

1.0.0

First release