Add `ajax/what-and-why` to capture some context about using Ajax

[ajpiano]

I think @liamks improvements to the main overview content (which now lives at ajax/dex.md are really solid. However, I think we should add a separate article's with @addyosmani's suggestions from #38 (below) to give people some fuller context.

• What is ajax? (more complete technical explanation). I don't think we're properly explaining just how much it can change how a developer looks at the flow of data in an application. Remember that we're talking about educating beginners (and later tackling advanced userS). More questions to address: why would I want to use Ajax? How does it help me?
• What's the problem with implementing Ajax without jQuery? Why is it more painful? (to a newcomer, a code snippet for how to implement this cross-browser vs using jQuery could perhaps be shown). This would definitely help them appreciate the benefits of abstraction.