Daily html report

Now you can generate the report of the daily computations with the command

$ oq-engine --make-html-report today

An example of output is here: http://performance.openquake.org/jobs-2014-11-05.html
See https://bugs.launchpad.net/oq-engine/+bug/1224816
Because it can be *very* slow, so by default we don't run that query. I will add a comment.