Merge pull request #932 from chrox/master

fix run out of memory when receiving large files from Calibre
fix #933
Merge pull request #934 from chrox/fix_933

fix #933
fix the nil tags data bug
Merge pull request #936 from WS64/master

fix the nil tags data bug
trace package loading process in koreader
refatoring: lazy loading of packages
move platform dependent files into 'platform' directory
Added acquisition relation types , fixes issue #940
Merge pull request #945 from evgsh/opds

opds / Added acquisition relation types , fixes issue #940
Merge pull request #944 from chrox/lazy_loading

refactoring: lazy loading of unused modules