Yellow arrow button added for pages in story to go to next page.
Button is shown only not on the cover page, on pages which have next page (not the last page, not fork page) and only if narration is turned on.
Button appears when last audio player on a page finishes paying, provided above conditions are true.
Merge pull request #48 from emilyems/ArrowButton

Yellow arrow button added for pages in story to go to next page.