CouchDB Revision ID generator
Based on:
- https://git-wip-us.apache.org/repos/asf?p=couchdb.git;a=blob;f=src/couchdb/couch_db.erl;h=81e97f2fe7f624015f1e01506afb0eef3eaac7e9;hb=HEAD#l653
- https://github.com/rustyio/BERT-JS
- http://www.erlang.org/doc/apps/erts/erl_ext_dist.html
- http://csm.tumblr.com/post/18963100318/how-couchdb-revision-number-generation-works
- http://stackoverflow.com/questions/5954864/how-does-couchdb-calculate-the-revision-number