(not ready yet) initial work on HAR support. See GH-60, GH-81.
HAR improvements:

* unique request ids (not based on URLs, not based on Python object ids!);
* correct request order;
* cleaner proxy management;
* information about requests is populated as soon as possible
getarg_bool utility
'history' argument for render.json. Fixes GH-60. See GH-81.
DOCs for 'history' argument
har GET parameter; some changes to make HAR validator shout less
fix "history"
minor cleanup
timings and sizes
render.har endpoint
more efficient header iteration
headersSize
show send time as 0 if it is insignificant
fix for aborted and incomplete requests
more timings (to make the display in HAR viewers nicer)
TST tests for render.har
DOC docs for HAR support
fix validation errors in edge cases
TST tests for HAR support in render.json
a simple UI for Splash