3.4/bugfix/use i18n translate

I think `kohana/core` should be a self sufficient module, not relying on `__()` function which is defined outside the module, in `bootstrap.php`.

I replaced occurrences of `__()` with `I18n::translate`. Also made some doc adjustments.

Kohana userland can still use `__()` if they leave the declaration in bootstrap. Otherwise they should use `I18n::translate` instead.

Please review thoroughly.