Should output protobufs instead of JSON

[kamahen]

Outputting JSON seems to be one of the most expensive parts of pykythe.pl (together with base64 manipulation), so for performance, it should output protobufs directly instead of using the entrystream utility). (SWI-Prolog supports this but performance should be checked because the implementation is mostly in Prolog (using DCGs + some small amount of C)).