Default plugin date parsing to YYYY-mm-dd, failback to mm/dd/YYYY

[akrherz]

Those of us on the wrong side of the pond like to specify dates as dd/mm/YYYY within the plugin.xml files used within Openfire plugins, but the ignite website wants the american format of mm/dd/YYYY.

Suggested change is to default to ISO YYYY-mm-dd when it senses a four digit number starting the string and then fall back to the previous default of mm/dd/YYYY. We can then update plugins going forward to use this more obvious string.