Fixup some of the Thread safety issues

Hi @auzroz, this addresses some of the Thread safety issues discussed at https://github.com/stefanpenner/country_select/pull/37

Use `Thread.current` for thread-safeness for now
  * This should probably be changed to use
    [RequestStore](https://github.com/steveklabnik/request_store) since
    I can imagine that `country_select` is used across so many servers
  * I removed `CountrySelect.locale=` because that should be handled
    with `I18n.locale`. I have, however, kept the getter and it just
    returns `I18n.locale`. This seemed the right choice from a
    testing/API-completeness perspective.

If you merge this PR in, it should automatically show up in your PR on stefanpenner's remote.