Add i18n translate alias function to bootstrap

Related to https://github.com/kohana/core/issues/526

When implementing I realised that I18n::lang() had to be set for the function to be useful, so in the end it did have to be placed after Kohana::modules().
Do we need to wrap it with `function_exists`? I know it does not hurt, but it's redundant since bootstraping is done only once.