add (content) creation time to evtxexport output

[joachimmetz]

Per https://blog.fox-it.com/2019/06/04/export-corrupts-windows-event-log-files/

• add (content) creation time (when present) to evtxexport output (mainly as UX improvement)
• describe corruption scenario in documentation

[joachimmetz]

Fix travis tests https://api.travis-ci.org/v3/job/547556325/log.txt