Merge pull request #175 from houqp/houqp-master

fix require error
Merge pull request #177 from houqp/houqp-master

update koreader-base for crengine update
update koreader-base
Merge pull request #178 from chrox/update_crengine

update koreader-base
fix libpng conflict in libk2pdfopt
fix nil pos bug in readerhighlight
Merge pull request #182 from chrox/master

fix nil pos bug in readerhighlight