Py3

I fixed codes to support py3. Only behavior of `__str__` method of `datum` is different.
In py3, `binary` object is converted to `b'xxx'`, but in py2 `str` is object of course converted to `'xxx'`.
Other differences are written in #33.

fix #33
I think, it is permitted in the style guide. And, its role is obvious, isn't it? (`u()` is inspired from `u''`, and `b()` is from `b''`)