Added hotfix for Plone 4.3.1

Seems Plone 4.3.1 has site_icon as a .png instead of a .gif

This just a small workaround (or hack if you wish) to allow people using this plone version to use the module while still supporting backward compatibility with the site_icon.gif.

I tried to make it a bit more complex using a extra by tal:condition to check "exists:context/site_icon.gif" but it only worked for the first instance of the img as the other already has a condition (tal:condition can't have 2 conditions apparently unless python: is used). So this was just a quick fix.

Another approach could be to "tal:define = cond1 ...; cond2 ..." and then use "tal:condition = python: cond1 and cond2" but ill leave that up to you to decide