datetime javascript tweak

I overrode the AdminDateTime to show the timezone, but there's this bit of javascript in grappelli that seems overly aggressive at removing the labels of the datetimefield.

Without fix:
![screen shot 2013-08-29 at 10 13 51 pm](https://f.cloud.github.com/assets/374396/1049404/94c91624-10a4-11e3-9e97-e6464b0b607f.png)

With fix:
![screen shot 2013-08-29 at 10 12 12 pm](https://f.cloud.github.com/assets/374396/1049394/48a86146-10a4-11e3-998c-bc265afdc8ee.png)

The "<" character shouldn't ever be in the label; It'd be escaped as "&lt;". I inserted a "<" into the "Time:" label while overriding the AdminDateTime and logged the resulting `a(this).html()`:

    def format_output(self, rendered_widgets):

        return format_html('<p class="datetime">{0} {1}<br />{2} {3}<br />{4} {5}</p>',
                           _('Date:'), rendered_widgets[0],
                           _('<Time:'), rendered_widgets[1],
                           _('Time Zone:'), rendered_widgets[2])


    Date: <input class="vDateField" id="id_event_date_0" name="event_date_0" size="10" type="text" value="2013-08-29"><br>&lt;Time: <input class="vTimeField" id="id_event_date_1" name="event_date_1" size="8" type="text" value="19:32:56">

As you can see the "<" gets replaced into "&amp;lt;" when rendered onto the template, therefore the proposed change to the regex will provide strictly better behaviour.

Of course, overriding the AdminDateTime is quite an unorthodox thing to do...