changed name of API from WorldStoryTree to Yggdrasil
YggdrasilStoryDescription convention is established, to allow returning POJO through jsonrpc4j without running into issues when
those objects are not exactly POJO but are objectify @Entity's with Ref in them, which Jackson has issues serializing for transit.

Overall, established the base Entity types for storing stories. Markov chain will be generated from these stories in a later revision
using mapreduce.