Remove pId (only one http request at a time)

Previously, if we clicked on a link and it was still loading, hovering over another link would preload this latter link in parallel.

This added huge (HUGE) complexity to the InstantClick’s equation, now in the rare case that a link is stuck, if we click on another one (or the same) it just redirects the user with location.href. Big win in simplicity, and a win in usability in the case that the page is stuck: clicking on another link gives back the browser’s loading indicator.
Bugfix: Scroll wasn’t remembered on popstate

Previously, if you went back then forward in history, the scroll would be at the top of the document instead of where you were.

Also, hovering over a link when this link is already in pHistory doesn’t override the scroll position anymore.
[Minor] Use currentLocationWithoutHash where appropriate
[Minor] xhr.open: remove optional async param
[Minor] !!arg_useBlacklist
[Minor] `triggerEvent` becomes `triggerPageEvent`
[Minor] Improve minified size + add semicolon at the end

Passing `document` and `location` as parameters to `InstantClick` shaves 6 bytes off the minified & gzipped file size. Yay!
Shaves 151 bytes off the minified non-gzipped size.
Simplify scroll handling on display

Gets rid of handling <a name="hash"></a> (HTML4)
[Minor] Comment about lack of href attribute
Remove displayed state

This “state” thing is confusing enough without an useless fourth state.

Also in this commit:
- Conditions in preload ordered in states’ chronological order
- Use currentLocationWithoutHash to check if InstantClick hasn’t been initialized already (so that we gain in minified file size)