Feature: maximum notifications onscreen

Added an option ("maxonscreen") to specify the maximum number of messages to display on screen.

The option can be set in $.pnotify.defaults and code was added to pnotify.pnotify_display that checks the length of jwindow.data('pnotify') and if necessary removes notifications (by calling pnotify_remove on items starting at the beginning of the array).

Let me know if you think this will be useful.

Awesome library!