Fixed CoolProp.State.State class for Python 3

Solved problems in `CoolProp.State.State` class on Python 3 as described in issue #81. The fix works for me on Python 2.7 and 3.3. 

Can someone please test if REFPROP fluids are working?  The type conversion for Fluid name arguments should be o.k. (`_add_REFPROP_fluid()` method takes Fluid name as C++ string --> it is passed to it by `add_REFPROP_fluid()` method as *bytes* --> Python byte strings automatically coerce from and to C++ strings)