Continue improving `ajax-and-forms`

[ajpiano]

@connormontgomery's improvements have been really useful, but this can probably use even more fleshing out. This could include some of the following improvements:

• "Ajaxifying" existing forms
• Form Plugin - to follow up the discussion in #38, I actually think it's worthwhile to show how the plugin can be useful in doing the above ("ajaxification") and how doing so provides a considerably more familiar flow to form authoring than just serializing data and shoving it off to a url you type into your JS: (eg. $.post("form_process.php", $form.serialize())
• Serialzing a form to an object (https://raw.github.com/cowboy/jquery-misc/master/jquery.ba-serializeobject.js, explain why this is left out of core (http://bugs.jquery.com/ticket/11002))
• Augmenting a form's data during a submit handler (for both traditional and AJAX submissions)
• Client side validation (all these changes might merit moving "validation" into its own article)
  • discuss how to leverage native techniques before just defaulting to using JS.
  • using a validation framework (jQuery Validation or a hand-rolled one) (in order to keep yourself from having the same phoneNumberRegex or whatever similar actual validation logic dupliated throughout your app)
  • use of native validation APIs ?
  • collecting and displaying client side errors (good vs. bad UX)
  • Using deferreds to incorporate AJAX-validation of form data on the client side with client-side only checks
    • Do some ajax validation and then do a normal form submit! (expand on http://jsfiddle.net/ajpiano/5tCp3/6/)

Stylistically, we should also move away from overly effusive phraseology ("extremely easy") and do our best to keep the tone focused on how it works, not how you'll feel as a user.

In addition, we should move the section on prefilters as per #62.

[arthurvr]

Also dropping a reference to https://github.com/jquery/learn.jquery.com/issues/566 in here. If we end up creating a series this could be part of it.