Fix issue to get portfolio items

Hello, 

There is an issue to get attributes of PortfolioItems. I get the following error.

  pyral.entity.UnreferenceableOIDError: PortfolioItem OID 12345...

The exception is raised at line 128 of pyral/entity.py.  

By looking at the code, I find getResourceByOID is failed because entity_name isn't properly set, and it is because the class isn't properly set in _basicInstance() method of EntityHydrator. So, I made the change to fix the issue.

Please take a look at and pull it if you like.

Thanks