Add 'alpha_dash', 'unique' and 'url' validation rules.

More infos at:
http://laravel.com/docs/validation#rule-alpha-dash
http://laravel.com/docs/validation#rule-unique
http://laravel.com/docs/validation#rule-url
Merge pull request #13 from tortuetorche/patch-1

Add 'alpha_dash', 'unique' and 'url' validation rules.