utils: Add some ISO 8601 forms to get_date()

Support the forms listed by the W3C [1].  I also removed the
'%Y-%d-%m' form, which can be confused with the '%Y-%m-%d' ISO form.
The new ISO forms can use 'Z' to designate UTC or '[+-]HHMM' to
specify offsets from UTC.  Other time zone designators are not
supported.

The '%z' directive has only been supported since Python 3.2 [2], so if
you're running Pelican on Python 2.7, you're stuck with 'Z' for UTC.
Conveniently, we get ValueErrors for both invalid directives and
data/format missmatches, so we don't need special handling for the 2.7
case inside get_date().

[1]: http://www.w3.org/TR/NOTE-datetime
[2]: http://bugs.python.org/issue6641