Add the built in validation rules

[indragiek]

These are defined as computed static properties instead of stored static properties on ValidationRule because the Swift compiler doesn't allow generic types to have stored static properties yet. This is nicer than putting these in the CommonRules struct like before, because now when an element initializer takes an array of ValidationRule<T>, you can use the shortened dot syntax to specify the rule. e.g.:

TextFieldElement(value: self.emailValue, continuous: true, validationRules: [.Email])

[dstnbrkr]

that's a nice syntax for rules!