Tweaked the implementation for faster rendering

Instead of mmaping the framebuffer, we now just prepare our image in
memory and do a write(); This seems to be much faster.

As a consequence, The user now needs to call Flush on the framebuffer
object to sync the changes to the video memory. (New method).
Added some documentation.

Specifically:
 - Added a summary comment to the Flush() method
 - Added a quick overview of how to use the package to the package
   comment.