Keep all times, show stddev in HTML; better runner loop

[belluzj]

@simoncozens I noticed when it was time for me to push that I worked on similar things to you, ie. the idea that the timed thing shouldn't be a random bash script but a single command.

I also did a few other things that you might find useful:

• keep all the times and show stddev in the HTML
• make the timed command run in a "work directory" that gets wiped between each run
• a better run loop that starts by enumerating all combinations and then reports progress (also to debug it you can make it stop after the first combination)

I didn't try merging just now because I'm off for today but I'll rebase next week on top of your latest work.

[simoncozens]

I’m not convinced about enumerating all combinations first. I notice a lot of time is spent shuffling around Python installations. I want to try reordering the list so that the project/version is in the outer loop and the setup script is only done once for each version, then the scenario is in the inner loop.