Added default view to load 'Invoices' when initially opening the 'Billing' module in ZPanel.

Hey buddy,

PS2Guy said he mentioned that it would be pretty cool to make it so the default view opens the Invoice page when the user initially loads the 'billing' module.

I've made the small change here and tested it, if you don't want to merge then thats fine but I've made the change if you wanted to pull :)

Cheers,
Bobby