koreader-nightly-20150318
New features：
- add support for GIF and PNG to picdocument provider
- add an option to show an arrow to indicate page overlap
- add PocketBook port of Koreader
- add FONTDIR env variable to customize font directory
- add stable OTA channel
- add support for Kindle Voyage
- add progress synchronization plugin
Enhancements：
- dynamic reader menu and config panel