fix kindle paperwhite2 support
reported at http://www.mobileread.com/forums/showpost.php?p=2696481&postcount=316
tested at http://www.hi-pda.com/forum/redirect.php?goto=findpost&ptid=1078988&pid=23393929
revert main GUI font to better-looking DroidSans
Merge pull request #368 from chrox/master

fix kindle paperwhite2 support