support python3. and add tox test.

I was a modification that supports both Python3 and Python2. I used [Six](https://pypi.python.org/pypi/six).

Other I have also supports things like the following.

- Between python2 and python3, I was made to be as much as possible the same coding style.
 - see also [this article]( http://methane.hatenablog.jp/entry/2014/01/18/Python_2/3_%E4%B8%A1%E5%AF%BE%E5%BF%9C%E3%81%AE%E3%81%9F%E3%82%81%E3%81%AB_%60unicode_literals%60_%E3%82%92%E4%BD%BF%E3%81%86%E3%81%B9%E3%81%8D%E3%81%8B)
- Add [tox](https://pypi.python.org/pypi/tox) test.  
- Updating .travis.yml. And measures the coverage with [coveralls.io](https://coveralls.io)
 - If you setup coveralls.io like [wootee-php](https://coveralls.io/r/woothee/woothee-php), it will be immediately start of measurement.

thx.