Implementation of IClientValidatable.GetClientValidationRules to enable client-side validation in Mkay.

MkayAttribute.GetClientValidationRules.cs

public IEnumerable<ModelClientValidationRule> GetClientValidationRules(
    ModelMetadata metadata, ControllerContext context)
  {
    var propName = metadata.PropertyName;
    var builder = new JsonBuilder(propName);
    var visitor = new ExpVisitor<JObject>(builder);
    Tree.Accept(visitor);
    var ast = visitor.GetResult();

    var json = new JObject(
      new JProperty("rule", _ruleSource), 
      new JProperty("property", propName),
      new JProperty("ast", ast));

    var rule = new ModelClientValidationRule
      {
        ErrorMessage = ErrorMessage ?? _defaultErrorMessage, 
        ValidationType = "mkay"
      };

    rule.ValidationParameters["rule"] = json.ToString();

    yield return rule;
}