Implement dynamic locale lookup.

Allow dynamic locale lookup and rerendering when the `application.locale` value changes.

* Requires 1.9.0-beta.3.
* Removes `t` utility function (it was not usable standalone).

Closes #1.

@bcardarella - This is a fairly significant refactor. Please feel free to close this PR and reimplement using this as inspiration on how to get the rerendering to work.
Does moving all of this into the helper imply that the values can ever only be used within the context of a template? I was thinking there are probably good use cases to want `i18n` values outside of templates (i.e. validation messages)