Added revel.Valid<VALIDATOR> methods for validation structs (#426)

These methods would remove some of the confusion when using the `revel.Validation.Check()` method from people new to the framework or validators, as shown in #423. The only struct which does not get its own method at this time is the `revel.Required` struct, mainly because of the weird name it would receive. All of the methods except for the `revel.ValidEmail()` method can be seen as alternatives to their structs. The reason why `revel.ValidEmail()` is special is because it uses the private `emailPattern`.