Proper customization group definition
Unify customization prefix

According to major mode conventions it should be clojure- instead of clojure-mode-.
Remove obsolete abbrev table definition

We get it for free from define-derived-mode.
Make the version const instead of var
Rename group to clojure in accordance with accepted Emacs style
Mark defcustom variables as safe
A few docstring fixes.
Remove unused prefix arg