Updated hook pre-commit to handle minification more efficiently

Here are the new features, according to our e-mail discussion :
* Won't minify the file if no changes were made and if it wasn't added to the commit (with git add)
* Will still work if CSSTidy / Jsmin are not available on the system -> Will just copy the files to the min locations, without actually doing the minification.
* Added two vars for CSSTidy and jsmin, to allow to change their location easily if they are not in your path.