Proof of concept for calling RServe from query actions

It adds a simple aggregation script (raggregation) that calls an R function
which calls the aggregate API and serializes the result as really ugly json

It can (only) handle date x medium queries at the moment, and the medium ids are fake

Json serialization now happens in R, but we could also see that as something generic
to build into python (convert 'sparse' table to highcharts json)?