Feature/navigation extended

Extended the navigation to allow PgUp to go backwards and PgDn to go forwards (to help compatibility with some clickers).

Added some basic paging to make presenting a little bit easier for anyone who wants a bit more control.

**Disclaimer:** I'm not that used to JavaScript, I did test in recent versions of Opera and Chromium, but if its rubbish code please do point me in the directions I should be looking :-)