Add page up and down key support

Hi!

Bespoke.js is completly awesome. 
I make HTML presentations for a long time, but I always needed to overwrite a lot of CSS style. That was before :)

So I made a presentation for a web conference in France.
The page down and up key support was missing, and that's what use the remote controllers.
I also added the Shift+Space key combination support, just because I like to use it.
I made some refactoring to improve code readability of this section.

I hope you will like it.

See you,
Thomas.