Use PyXML for iso8601 date on RHEL5 and dateutil after

RHEL5 doesn't have python-dateutil. F17+ doesn't
want to require PyXML. We need to parse iso8601
dates. dateutil approach is better, but can't
be used on RHEL5.

On RHEL5, require PyXML. on RHEL6+, require
python-dateutil

Also changed parseDate to parse_date to match
our naming convention.

parse_date moved into new file isodate.py, which
will use whichever iso8601 date parser it finds,
preferring daetutil.

Note: the date overflow related test cases now
determine which impl is being used when they
verify the results. Either answer is ok for our
purposes.

add isodate to spec

fix test case references to ServerTz

update spec comment about dateutil versions
Merge pull request #516 from candlepin/alikins/dateutil

make python-dateutil optional for rhel5