Themeable faces

It's not that the faces weren't configurable before but they weren't
really themeable.  This patch makes the mode more consistent in it's use
of faces for theming and customisation.
Conformity with package.el format

This is in preparation for publishing on melpa.
Merge pull request #11 from behaghel/master

Conformity with package.el format
Minor tweak to README.
Merge pull request #9 from russell/master

Themeable faces
Avoid divide by zero where document is empty or small
Be explicit about guarding against divide by zero
Merge pull request #14 from mneilsen/master

Avoid divide by zero where document is empty or small
Change description, add thanks
Fix typo in acknowledgements