PMX importer: Set material property "use_cast_shadows" that is available in BLender 2.71 or later.

To keep backward compatibility, "use_cast_buffer_shadows" is still used.
PMX importer: Fix file path issues on Linux and OSX (case-sensitivity and path separators).
Merge pull request #21 from iRi-E/support_2.71

PMX importer: Support 2.71 and fix texture file path issues