! Fix a bug discovered/reported in #277
ensure I18n is initialized for Node/CommonJS only when the environment is a Node/CommonJS env
Merge pull request #279 from myoung8/ensure_node_env_for_factory

ensure I18n is initialized for Node/CommonJS only when the environment is a Node/CommonJS env
+ Add class FallbackLocales
+ Add spec & code for merging fallbacks
* Set default value to true
~ Modify CHANGELOG and README for the new feature
[ci skip]