Allow unicode names for structs in Python 2.
Allow unicode names for structs in Python 2.

Fix for issue #25. Now allows unicode names in Python 2, in addition to
binary string names. Python 3, as before, only allows unicode names.
Patch passes all tests in 2.7 and 3.3.

gkonstantyno/construct,riggs/construct,riggs/construct,0000-bigtree/construct,mosquito/construct,gkonstantyno/construct,mosquito/construct,0000-bigtree/construct