[MRG] HAR support take 2

Here is a follow-up for https://github.com/scrapinghub/splash/pull/107. 

* HAR entries are grouped into pages; 
* 'wait' meaning is changed (it restarts after each redirect);
* loading of resources is cancelled earlier;
* more tests are added.

Todo:

* [x] add more tests;
* [x] add tests for timings;
* [x] fix an issue with 'loadFinished called multiple times' when /delay?n=2 is rendered with timeout=1, wait=0.5
* [x] ~~redirect chain can be grouped to a single page: it looks incorrect~~ (it seems there is nothing wrong with it);
* [x] rewrite entity grouping;
* [x] ~~rewrite redirects handling code~~ - let's keep it in mind, but it is not required for this PR;
* [x] ~~drop history GET argument?~~ - it is a separate issue