Updated version to 0.15
NPM requires x.x.x style version number
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
Merge pull request #176 from becka11y/feature/landmarks172

Fixes #172 ARIA landmarks
Merge branch 'develop' of https://github.com/readium/readium-js-viewer into feature/visibilityPrevNextButton
- update submodules