Added an extension point to hook in third party persistence layers

[rsaccon]

For page loading I have renamed $.ajax to $.ajaxExt, so it can be selectively overwritten by third party caching and persistence layers, as e.g. persistencejs, which provides sync capabilities between local HTML5 SQL DB and serverside DB.

I have also provided a working demo based on persistencejs / jquery-mobile plugin and with patched jquery at my persistencejs fork, just drop the whole project into a server and load demo-jquerymobile.html, at first load pages get loaded via ajax request and then cached into local HTML SQL DB.

If another way is preferred, to allow overwriting ajax page loading, just let me know and I will adapt the plugin and demo.

[rsaccon]

One more thought:
in case the suggested way to hook in a persitence layer for page loading (using custom-named $.ajax) is not appreciated, another way would be to decide based on URL whether to reroute to persistence layer. That would require no modification of jquery.mobile.js

[rsaccon]

modified my demo and plugin according to previous comment. No more jquery-mobile patching necessary, please discard this pull request.