Fixed: highlights are now generated on pages that are partially visible when HighlightOnPage is called

-Added "isPageInViewport" function to diva.js as public wrapper for isPageVisible.

-Added space to highlight div class to allow for external classes to be added.

-Changed highlightOnPage function to initially highlight all visible pages as opposed to only getCurrentPageIndex()

-Removed Events.isSubscribedTo function as its existence was only warranted by my not understanding how the "handle" return value of .subscribe worked; the user will have the handle object if the function has been subscribed.

Resolves #175
Resolves #176
Fixed: temporary resolution for deleting highlights with non-default classes

There's likely an easier/better way to do this, but this works temporarily.

Refs #173