Add support for "fallbacks" plus refactoring

* Add pluggable support for localization/translation "fallbacks"
* Add tests to cover "fallbacks" delegation from "I18n" to "SafeI18n"
* Move "SafeI18n" specific logic into the "SafeI18n" namespace rather
  than cluttering up the "I18n" namespace
* Use the current "locale" rather than english in "i18n_spec"
* Other miscellaneous refactorings
* Update documentation
Version "0.2.0" complete (major feature: fallbacks)