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.
I'm not sure what you mean by "hitting the db" -- if the validator's doing its job, I'd imagine it's not being persisted to the db. I agree that `@event.should be_valid` would be better, but I'm following suit with the rest of the specs.