Add UI buttons to the pages

[ronw98]

I added 3 buttons (that can be shown or not):

• A skip button that automatically calls the onRightOut listener
• A previous button that goes back one page
• A next button that goes forward one page

The text of the skip button is customizable The next and previous buttons are ImageButtons with an arrow drawable. Its color can be changed in API 21+

I also added a builder for readability purposes.

These changes are compatible with the previous version of the library.