Add conf file for slow_benchmarks

[Bjorn]

Now that #34 is merged we should add a config file for asv using the slow_benchmarks/ folder.

And for the ordinary benchmarks (benchmarks/) we could also set the timeout to something considerably smaller than the default of 60 seconds (a target of 2 "travis-seconds" was suggested, but since people may be using old hardware maybe 8 seconds or so is reasonable to put in the default config).