prevent recheck of multiple radio buttons

bug detected when using validation **"required"** on group of **radio** buttons with same name attribute, i.e. "payment_method"

when not valid (i.e. no radio button was checked), an error message span would be created for each radio button element.
![untitled-1](https://cloud.githubusercontent.com/assets/206041/4136846/bebfbc20-3381-11e4-91b3-679086faf53e.gif)

this patch adds a memory array named **checkedInputs[]** 
to hold the names of elements that have been checked 
during the iteration of isValid() func,
thereby preventing a recheck of elements with the same name, 
in the situation of radio buttons.

during the iteration through each form element, each element is added to this array before it is being checked.

if the element name already exists in the array, the validation of that element will not be performed.

since each form element has unique names, except for radio buttons and checkbox groups, this should not affect other input types.


**Important Notes on Multiple Checkboxes:**

when using validation on a group of checkboxes, 
set validation rule to **checkboxes_group**, and specify qty attribute, i.e. **min1**

otherwise, the keyup event will trigger the validation on the checkbox, 
without being able to read the other checkboxes in same named group,
and you will see an error message, even though the required qty of checkboxes is checked