add fallback locale support

[saknarak]

• Add fallback locale support. When you have a new translation, no need to completely translate to new language. Just leave untranslated message fallback to base language.
• Modify the test script by remove the GMT part Because of my timezone is +07:00, so it not pass the test when specified timezone.

[martinandert]

Thanks for your contribution. I have cherry-picked the stuff that I liked and released a new patch version. What's different from your PR:

• I don't think there's a need for emitting a fallbacklocalechange event.
• The global fallbackLocale is set to null by default.
• The fallbackLocale can also be provided as an option to translate.

I have also tried a different approach to make the tests independent of the timezone set on the testing machine. I hope this works for you. Let me hear if not.