Feature/i18n yml fallbacks

Replace #226, #114 
Differences:
- default `fallbacks` is `true` now (if not set in config file)
- added spec and doc for `fallbacks: :default_locale`
- removed unused helper method `stub_config`
- did not add `rspec-its` since it's not necessary yet (although it won't hurt)

See what else should be fixed/changed before I merge it on Friday.

cc @johnnyshields @Deradon