New context manager: force_timezone()

[fabfuel]

As useful as forcing a specific locale, it is useful to be able to enforce a specific timezone.

In my case, I want to render a template for a specific user who has a preferred timezone, but this is not a logged in user, why it would be too complicated to implement this via a request context aware timezoneselector.

The implementation and behaviour of force_timezone() is similar to the existing context manager force_locale().

[rnd0101]

Does this repeats the same mistake as force_locale, which makes this useless in multi-threading environment?