add npm package "rethinkdb" (v1.14)
add RethinkDB persistence back-end

* adjust persistence back-end API
  * make 'init' and 'del' async, for consistency and to be able to
    test/benchmark without the wait.for/fibers dependency
  * 'read' is still sync (see docs in pers.js for the reasons)
  * add 'close' function for clean shutdown
* add default RDB back-end configuration; pass config to back-end
  module on initialization
* adjust tests and back-end mock according to 'init' function changes
* initialize persistence layer during server startup
tests and benchmarks for RethinkDB persistence back-end
Merge pull request #3 from yelworc/pbe-rethinkdb

add RethinkDB persistence back-end