Fix url validator

Calagator currently has an [event up](http://calagator.org/events/1250466668) with a broken link in the "Website" field. The url validator allowed an invalid url of the form `example.com, http://example.com/` to be persisted, because the string contains `http`. Fixed by adding a `^` to the validator regex. Tests for valid/invalid urls are included.
Is this hitting the db? I think calling `@event.valid?` would be better for these specs anyway.