Use dateutil for parsing timezone

datetime.datetime.strptime() uses time.tzname to detect timezone
abbreviations ('EDT', 'CST', etc.).  Unfortunately, these names
are system dependent.  So, on my machine which is set to central
time, strptime() won't parse a time string with 'EDT' in it.

Using dateutil.parser works around this, though I'm not thrilled
about adding a dependency for this little thing.
Merge branch 'master' of https://github.com/openelections/openelections-clarity