Merge pull request #99 from onyx-cap/tagus

hide sync button if have no isbn number in reader.
fixed the problem that metadata can not be found after reboot.
Revert "use different way to compute MD5 in case of the calculation is not identical after reboot."

This reverts commit 4cbff90a64bfd1b86bc05e097efa7d90eba8c7ce.