Support for bullet face override

Sometimes bullets don't look good in the `org-level-x` face and so this
change allows an override face to be specified with
`org-bullets-face-name`. Set this to the face name as a string to force
a particular face to be used for bullets, or set it to nil to use the
normal Org-mode heading face.

I also cleaned up the code to determine the Org-mode face name for the current heading based on this tip: http://stackoverflow.com/a/1242366/1533863, although it's possible the previous, more complex, code that determines it was there for a reason.

This can probably be achieved in a cleaner way, but I'm new to elisp...