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.