Added popoverContentSize property to match UIPopoverController behavior

The way WYPopoverController is currently implemented, you cannot change the size of the popover controller dynamically, you have to have set it before the content view controller is initialized.  UIPopoverController has a property, "popoverContentSize" that allows you to change or set the size of the content view anytime, even after its displayed.  I added this property to WYPopoverController to give the same behavior.