Grow "Ajax Events" section into discussion of global Ajax handling

ajax/ajax-events.md
From @addyosmani: "This is another example of where pulling in examples we have in the API site would really help. Even if we were to just take out the code snippets from http://docs.jquery.com/Ajax_Events and add a lot more narrative around them to explain things that would be better than what we have right now. "

This should just grow to discuss how to handle ajax globally - with both ajax events, as well as prefilters, transports and converters. The content from ajax/ajax-and-forms.md from @connormontgomery about prefilters should be moved here.

In addition, discussion of using your own app-level request mediator instead of just trying to do everything through jQuery's events should probably be here as well.

Start with ajax/ajax-events.md and ajax/ajax-and-forms.md, then compare their coverage with the Ajax Events API material at docs.jquery.com/Ajax_Events. Done means expanding the global Ajax discussion with examples and narrative for events, prefilters, transports, converters, and app-level request mediators, while moving the prefilter content into the Ajax Events section.