Changed State class to take Fluid name as str instead of bytes

- eliminate bytes/string conversion problems under Python 3
- consistent interface for Props and State classes
Resolve Python 3 problem with StateDict.items() (Fixed #81)

- items() method for dict returns list on Py2, but dict_item on Py3
Merge pull request #86 from spinnau/fix_81

Fixed CoolProp.State.State class for Python 3