Make date format for scheduler configurable.

Currently, the revisioning scheduler forces you to use dd-mm-yyyy format for the publish date. This can be problematic if you are trying to use a datepicker plugin, or simply trying to make it more user friendly for places like the United States where another format like mm/dd/yyyy is more common. This patch allows you to set the date format from the settings page. I have it marked as "needs work" as my current solution relies on the DateTime class and will only work on PHP 5.3 or later, which may be a problem for a general release. If someone knows of a workaround they are willing to contribute, that would be great!