Better class feature, and better API

• In respond of request #19, now you can fill the data-scroll attributes with as many classes as you need, so it's compatible with any CSS framework.
  If you leave it empty, the class is-outside will be used by default.

• Better API: now you can test if an element is isInsideViewport, isRepeating or get repeatingCount.
  Go check the API section in the documentation.