support origin=any distroseries=natty in the review-stats call

Adding support to return stats across origins is trivial, but two related points:

1) We could return stats for the same package from different origins,
2) the resulting json does not (currently) contain any info to let you know which stat is for which origin (or distroseries, for that matter)

Is that OK, or do we also need to add origin or distroseries when origin or distroseries (respectively) is 'any' (or just include them always?)

I think thats ok for natty because we do not have many origins, just archive.ubuntu.com, archive.canonical.com and the for-pay PPAs. There should be no overlap there in the packagename (as this will cause other problems too).

But once we expand it into PPAs we need either to include the origins information in the stat json or aggregate on the fly. But my gut feeling is that when we open this up for all PPAs we need to incldue something like a fake origin "default-system" or somesuch that will include "ubuntu,canoncial,all-for-pay-ppas". And make additional calls for each enabled PPAs. This way we get away with just one call to the API for most users.