Merge branch 'p2' into 'master'

Fix update crash in case of unset dates in APK; add an option to use APK's entry date

I encountered an error when I tried to add [TrackID APK](https://play.google.com/store/apps/details?id=com.sonyericsson.trackid) (files in the APK don't have dates set): as far as I can remember `datetime(*manifest.date_time)` was raising `ValueError`.

Also add an option to use date from APK as this APK's entry date (may be useful in case of a simple binary repo): `fdroid update --use-date-from-apk`.

I should have split this commit, but I hope this would be OK too.

See merge request !104