Add dateutil and use it to parse dates.

Update _safe_get_element_date to use dateutil instead
of the builtin parsers which aren't as good.
Add test for products with obscure dates.
Update requirements.
Update README to include new dependency.
Fix dateutil dependency.
Return a date, not a datetime.
Merge pull request #33 from adamlwgriffiths/dateutil

Add dateutil and use it to parse dates.