Update slug.php Observer. Uniqueness as option

I use slugs very often simply to give the end use a better description of the URL and for SEO.

Ex.  "http://server.com/doc/281/communications/about/about-the-communications-department/"

In a case like this I would prefer not to have uniqueness in the slug as I only require the ID (281) and I dislike slugs with numbers in them (about, about-1, about-2) since many "departments" in this case will have an "About" page.
Slug uniqueness as option
Merge pull request #342 from mikepmtl/patch-4

Update slug.php Observer. Uniqueness as option
on new objects, validation should run on all fields