Support multiple forms in one function call

Example:

    <form id='subscribe-discount'>
      <input type='email' required>
      <input type='submit' value='Subscribe'>
    </form>
    <form id='subscribe-new-products'>
      <input type='email' required>
      <input type='submit' value='Subscribe'>
    </form>

In current version, you have to invoke `validator` twice:

    $('form').each(function(){
       $(this).validate(options)
    })
    // or
    $('#subscribe-discount).validator(options)
    $('#subscribe-new-products).validator(options)

If you do this: `$('form').validate(options)`, will fails silently.

This patch makes it works more like jQuery: treat itself as an array, and return itself.