[Minor] Refactoring changePage

+ Puts triggerPageEvent outside of instantanize
+ Add comment regarding replaceChild
[Minor] Rename useBlacklist to useWhitelist
Code cleanup, + bugfix for one regression

Bugfix:
- Regression introduced in 091d55d346975f5c7bad93f3dc3c754aa0b014cf , would prevent scrolling when clicking on a link with an #anchor

Code cleanups:
- Removing redundant conditions (!p.isPreloading || p.isPreloading && …)
- Removing unused `e` variable in `readystatechange`’s arguments
- Refactoring: add function `setPreloadingAsHalted`
- Go through `init`’s arguments backwards (better for gzipped file size)
- Some comments makeover
- Check if InstantClick has already been initialized now the first condition in `init`
- Removed `supported` and `useWhitelist` from `debug`
- Better names for `triggerPageEvent` and `on`’s arguments
Debug function is now in comments
Rename “global” variables

This shaves off 100+ bytes from the minified+gzipped file size.