added the play/pause button state that was removed by mistake when migrating from bitmap spritesheet to scalable icon font.
Media Overlays volume control mute state (icon)
mute/unmute control
accessibility: added missing i18n language strings (Media Overlays control + page previous/next), and added fallback mechanism to en-US (+ console log for missing strings)
added Media Overlays button to disable / enable "touch-to-play" (so that clicks on content documents do not interfere with other features, such as annotation or dictionary)
Updated about dialog with new wording and version number
Merge pull request #31 from readium/develop

Merging in latest stuff from develop