HTML.Nofollow enabled

added nofollow rel attributes external links in snippet's description.
More detail packages specs http://htmlpurifier.org/live/configdoc/plain.html#HTML.Nofollow

Note: i don't test this, please check before merge.
Merge branch 'patch-1' of github.com:mayoz/laravelsnippets into mayoz-patch-1

Conflicts:
	app/views/snippets/show.blade.php