Fix for error message translation (issue 332)

Defer the call to I18n.t for 'error_messages.email_invalid' because the I18n.locale may change between class initialization time and run time. This is definitely true in my app which handles users from multiple locales simultaneously, changing the locale for each request.

I'm sorry I could not provide a decent test for this; I experimented with tests for about an hour but could not find a clean approach.

I've tested this on Rails 3.0