Fix PSR0Resrouce to be compliant with the PSR0 standard.

* Each _ character in the CLASS NAME is converted to a DIRECTORY_SEPARATOR. The _ character has no special meaning in the namespace.
Can you please add a feature for the case where underscores exist in the package namespace? PSR-0 does not convert these and I want to make sure we're explicitly handling that case

e.g. CodeGeneration\SpecExample\TextMarkdown -> CodeGeneration/Spec_Example/TextMarkdown.php