added events: enter_keypress, focus, blur; added keyboard navigation: pageup, pagedown

I hope I've done it OK, my first pull request :)

page up / page down is moved fixed +-5 items, I didn't find any property which controls dropdown size.
enter_keypress: I am controlling it on keypress, since I had issued within keydown event

And I have no idea on how to exclude dist folder from pull request:(