added new datetime_from_dir() function to parse BackInTime folder names

[mdhoney]

Back In Time uses rsync+hardlinks, but (on my machine at least) the timestamps get messed up sometimes, so I needed a more reliable way of getting the original timestamp from the directory name (e.g. 20200428-170001-777). It's mostly just adjusting the strptime format, as well as converting the time to UTC (since the folder times are in local time).
To actually use this, a small change in rsynchl.py will be required.

[mdhoney]

I didn't realize that committing to my branch after initiating the PR would add these changes too. Hence the force-push in order to reset to previous commit.