Add benchmark for spray

[jrudolph]

Currently, only the json benchmark is supported.

[jrudolph]

If merged, this would fix #80.

[bhauer]

Thanks for the contribution, @jrudolph!

[jrudolph]

We just found two or three things we'll have to update. @sirthias will also give his LGTM when we are finished.

[sirthias]

Ok, this should now provide a nice approximation of how spray would currently perform for the JSON benchmark.
We'll see whether we can add the required bits for the other tests as well later, but for round 5 this is all we can currently supply.
We are quite curious as to how spray will fare, so it'd be great if you could get this PR into the suite before round 5 is started.

Please let us know if you need any help with setting the right attributes for spray! (We couldn't find the data origin for these values (classification, etc.) in the repo...)

[pfalls1]

Thanks @jrudolph and @sirthias. This looks good.