Better pjax:click handling, X-PJAX-Vars response header added, some small improvements

In case pjax:click event changed the link, reread it before request.

If scrollTo is false, at least bring the target container in viewport.
options.request_time added.

Server now can redirect user to a new location at pjax request, as it
would at page load with `X-PJAX-Location` header.
options.redirect_handler can handle the redirect nicely.

Server side variables sent as X-PJAX-Vars header in JSON format are
stored with state in $.pjax.state.vars.