support for arrow navigation (forward and backward in slides) and any-level topics

Hi Nic,
Thanks for your simple emacs presenter.. I just made some modification, as listed below:
- <f5> to toggle minor mode (PRESS minor mode, or ORG-MODE)
- right arrow to go 5 lines forward OR until next topic (the one that come first)
- left arrow to go back on line at a time
- esc + left /right - back/forward to the previous/next topic directly