Added seconds countdown if "counterElementID" is set

[mwhawkins]

This adds a counter to count down the number of seconds remaining until expiration if counterElementID is set, e.g.:

<h3>{% trans 'Your session is about to expire in' %} <span id="in_seconds"></span> {% trans 'seconds' %}</h3>

<script type="text/javascript">
            var sessionSecurity = new yourlabs.SessionSecurity({
                pingUrl: '{% url 'session_security_ping' %}',
                warnAfter: {{ request|warn_after|unlocalize }},
                expireAfter: {{ request|expire_after|unlocalize }},
                confirmFormDiscard: "{% trans 'You have unsaved changes in a form of this page.' %}",
                counterElementID: "in_seconds"
            });
        </script>

[its4net]

This would be very helpful. Please merge.

[mpasternak]

I love it! @jpic , what do you think?

[jpic]

Working great, enabled by default, except made sure we don't introduce a new translatable string, merged manually