Make sure the collection id is specific to the backend selected.

If the user first generated an output using (say) the rdf2text backend,
we'd return that output even if the user tried to generate a PDF for
the same article.  I believe this is an artifact of the original
design, where we planned to cache the bundler output separately.
But at the moment we only cache the final output, so the collection
ID needs to be specific to the backend chosen.

Change-Id: Ic6a55b9ac160a389a8a612fde87676752331590e
Clarify README with local development instructions.

Change-Id: Ic61ff6df80e22e2e8977e7b9cd44fe58aff328cd
Add a human-readable help page to quickly test/check the service.

Change-Id: I7b6577387ac9ab57e16b63bc7001ce6437855012