replace occurances of `__()` with `I18n::translate`

Replaced occurances of `__()` with `I18n::translate` in Exception and
Validation, as well as the view files error.php and stats.php
Various doc changes regarding `__()` and `I18n::translate`
Merge pull request #535 from kohana/3.4/bugfix/use-i18n-translate

Use `I18n::translate` instead of `__()`