fix(form): isvalid should not delete error state and made optional now

the is valid behavior deletes possible existing error states on a field, although it shouldn't.
That's because the field validator always removes the error state by default (before probably adding it again) and did not respect a showErrors setting. showErrors however is only meant to show new errors. If it is false it should not touch any existing error state class.

I now also added the showErrors parameter as an option to the is valid behavior in case someone definately wants to show errors on not already validated fields.

As the parameter is optional the change is fully backwards compatible.

src/definitions/behaviors/form.js

@@ -338,18 +338,18 @@ $.fn.form = function(parameters) {
           blank: function($field) {
             return String($field.val()).trim() === '';
           },
-          valid: function(field) {
+          valid: function(field, showErrors) {
             var
               allValid = true
             ;
             if(field) {
               module.verbose('Checking if field is valid', field);
-              return module.validate.field(validation[field], field, false);
+              return module.validate.field(validation[field], field, !!showErrors);
             }
             else {
               module.verbose('Checking if form is valid');
               $.each(validation, function(fieldName, field) {
-                if( !module.is.valid(fieldName) ) {
+                if( !module.is.valid(fieldName, showErrors) ) {
                   allValid = false;
                 }
               });
@@ -1235,7 +1235,9 @@ $.fn.form = function(parameters) {
               module.debug('Field depends on another value that is not present or empty. Skipping', $dependsField);
             }
             else if(field.rules !== undefined) {
-              $field.closest($group).removeClass(className.error);
+              if(showErrors) {
+                $field.closest($group).removeClass(className.error);
+              }
               $.each(field.rules, function(index, rule) {
                 if( module.has.field(identifier)) {
                   var invalidFields = module.validate.rule(field, rule,true) || [];