Allow custom classes on OK and Cancel buttons

- This makes it significantly easier to use vendor styles (e.g. Twitter
  Bootstrap) on those buttons
Merge branch 'button-classes' of git://github.com/mark-rushakoff/best_in_place into mark-rushakoff-button-classes

Conflicts:
	lib/assets/javascripts/best_in_place.js
Merge branch 'mark-rushakoff-button-classes'
Depend on rails again