Fixes #172 ARIA landmarks

Added main landmark to epub content region.
Changed toolbar region landmark to banner from navigation
Added a region named Page Navigation to prev/next button area
Added Page Navigation to EN-US messages.json
Added empty alt attribute to empty library arrow image

I consulted with other folks and we felt banner was a more appropriate role for the toolbar than navigation. I didn't make the page navigation area (with the prev/next arrows) a landmark but did mark it as a region. This does require an additional string, Page Navigation. I added that to the en_US file. 
No, this image has no meaning to screen reader users so I made the alt text
empty. I could have described it as pointing to the add to library button
but that seemed like overkill.

-becky


On Mon, Aug 25, 2014 at 3:04 PM, ryanackley <notifications@github.com>
wrote:

> In templates/empty-library.html:
>
> > @@ -2,5 +2,5 @@
> >  	<div id="empty-message">
> >  		{{strings.i18n_add_items}}
> >  	</div>
> > -	<img id="empty-message-arrow" src="images/library_arrow.png">
> > +	<img id="empty-message-arrow" src="images/library_arrow.png" alt="">
>
> Did you mean to put the text of the message here?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/readium/readium-js-viewer/pull/176/files#r16673744>.
>