fix localization tests for 1.8.7-p302 (wtf)
The I18n::MissingTranslationData exception escapes key names for its html_message

Also added deprecation message for the :rescue_format option
Actually make the empty test file empty (to stump Psych as well as
Syck). Closes #114
Add I18n.locale_available? and enforce available locales

I18n.locale_available? looks for the passed locale in the `I18n.available_locales` list.

---

Enforce available locales

When `I18n.config.enforce_available_locales` is true we'll raise an
I18n::InvalidLocale exception if the passed locale is available.

The default is set to `nil` which will display a deprecation error.

If set to `false` we'll skip enforcing available locales altogether (old behaviour).

This has been implemented in the following methods :

- I18n.config.default_locale=
- I18n.config.locale=
- I18n.translate
- I18n.localize
- I18n.transliterate
Bump version to 0.5.1