Move to assets lang system

Since 1.6 lang files are automatic detected in the folder : assets/modid/lang
Since 1.7, LanguageRegistry is obsolete.
The change is necessary, it will be more convenient for translators (vanilla lang files use UTF8 without BOM, and actually buildcraft use ISO 8859-1).

it really closes #1174 and also fix #1435

However, it will break all existing translations, I'll take care of them tomorrow if this pull request is accepted.