fix #12

fixed the unicode issue. 
However the tests are still failing for another reason :
If I apply this fix on the commit 3aaf672086aa20e44cb810acb5cc071baa989c2b ( Jul 01), the tests become fixed.

The new issue occurs now in prov/model/__init__.py in add_extra_attributes at self._extra_attributes.update(attr_list)
the attr_list seems not to be hashable or contains elements that are not hashable.

May be you can provide some hint on that issue