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.
Add PSR-0 underscore support to the specification file. Refactor and more tests is still needed.
- Refactoring the PSR-0 reverse classname algorithm;
- Remove the usage of native PHP function for filesystem interaction (replaced with the usage of the Filesystem class) to allow the PSR-0 compliant to be tested;
- Update of the PSR0Locator specs to reflect the new way of creating a resource;
- Adding specs to the PSR0Locator to verify that loading of class containing underscores in the name is compliant with the PSR-0 standard (classname `spec\PhpSpec\Some_ClassSpec` must map to the file `spec/PhpSpec/Some/ClassSpec.php`).