Add native support beforeSend and fix for asynchronous requests

[fourteenmeister]

Two commits:

1. Native support beforeSend
   Example:

$.pjax({ beforeSend: function(e, object) { // some scripts return true; }, container: "[data-pjax-container]", });

1. adding a script tag, depending on the type of request
   Example:

$.pjax({ async: true, // by default container: "[data-pjax-container]", });

will append script tag asynchronously

$.pjax({ async: false, container: "[data-pjax-container]", });

will append script tag synchronously

[mislav]

Could you please explain the rationale behind this change?

[fourteenmeister]

first commit - Of course I know that there is an event 'pjax:beforeSend'. But it is hung worldwide! What if I want to go to some particular query or queries to execute the something script before send query?

[fourteenmeister]

second commit - there are situations when the script does not have time to load (due to the fact that it is loaded asynchronously). Some scripts are already starting to work immediately after the request and an error occurs (the script did not have time to load). This commit fixes the problem, of course if the async attribute set to false

[mislav]

Sorry it took me long to review this pull request. I had to give it a lot of thought.

I don't see it worth going forward with these changes, though. First of all, this PR deals with 2 separate features, and would be better if it was submitted as separate pull requests. Second, I'm not in favor of both of these solutions.

1. You manually call options.beforeSend() before starting the ajax request, but jQuery's beforeSend callback is supposed to be called with 2 arguments: xhr object and ajax settings hash.
2. We don't want to support async: false ajax scenarios because it's not in our interest.
3. We're aware that using pjax can mess up sequential loading of per-page external script resources. This is an unfortunate technical limitation due to the fact we can't rely on eval External scripts are evaluated async #331 (comment)