use CI to automatically update benchmark website on new data

[orbeckst]

astropy does something clever (or confusing): the master branch are the actual benchmark tests (our mdanalysis/benchmarks), their results branch contains results (our benchmarks master), and any commits to their result branch trigger Travis, which automatically deploys the asv web site page to the gh-pages branch. (#2 (comment))

This setup has the advantage that anyone can easily add results from their own favorite machine (the results are stored in a results/MACHINE directory in the results branch) and the benchmarks website will immediately update with the merged data.

[orbeckst]

In 94fcf3f I let the cron.sh pull the results so we should automatically merge everything once a day; assuming that this works, this issue should not be super-urgent.

Otherwise one can always manually update using

git pull
asv gh-pages --no-push
git push origin +gh-pages

[orbeckst]

not relevant anymore