[WIP] 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:

* [ ] add more tests;
* [ ] add tests for timings;
* [ ] fix an issue with 'loadFinished called multiple times' when /delay?n=2 is rendered with timeout=1, wait=0.5
* [ ] rewrite entity grouping;
* [ ] rewrite redirects handling code;