altered interpretation of environ

It's been hard to use environment variables with nested configuration because linux shells don't allow the dot character in identifiers.  This change makes configman interpret os.environ differently.  Any doubled underscores in environment variables where the key is not all uppercase, will be interpreted as the dot character. 

"resource__postgresql__database_name" would be interpreted as "resource.postgresql.database_name".  this will make configman apps compatible with "export" as well as "env" methods  of setting environment variables.  
Why not ``if '__' in k and k != k.upper():``? It's exactly the same, right?