Only enable auto-update of slug when page is first created

Best SEO practice: page URL's shouldn't change often if at all.
Merge pull request #2198 from mikezilla/2.1/feature/sticky-slug

Only enable auto-update of slug when page is first created
Upgraded Google Analytics API to v2.4.
Merge branch '2.1/develop' of git://github.com/pyrocms/pyrocms into 2.2/develop

Conflicts:
	system/cms/modules/pages/views/admin/form.php