Unicode handling fixes.

Return all EXTH properties as unicode text by default.
Check for file jumps after correcting timezones.

The previous code would incorrectly detect a "jump" between two files if we'd
seen a timezone change but the first line of the new file was still logging in
the old timezone.
Allow backwards jumps across files.

This was only ever a sanity check that I'd never hit, now I have log examples
that start at the epoch and this check prevents their processing. Treating them
the same was we treat jumps back to the epoch within a file seems to work out
fine, so I see no need to keep this overly cautious check.