fix #127 fix cover image metadata in OPF (for iBooks)
fix #127 fix cover image metadata in OPF (for iBooks)

kuad9/review_study,murashitas/review,krororo/review,suzuki/review,kmuto/review,masawada/review,kmuto/review,krororo/review,krororo/review,orangain/review,masawada/review,suzuki/review,murashitas/review,kmuto/review,takahashim/review,kuad9/review_study,orangain/review,takahashim/review