Python six compatibility layer, as requested

[KM4YRI]

As requested per #9.

Regarding other python 3 paradigms and backporting to python 2. enums are possible using 'enum34', a backport library to add enum syntax to python 2. I didn't want to add this without discussing, it's your call, but personally it's probably not worth the hassle of yet another dependency. I didn't see a clean way to get 'yield from' syntax in python 2, but what's there now works for both. All in all it seems to be in pretty good shape. I've got my build system setup to test both python 2.7 and 3.5 and it seems to be working with both.

[dnet]

Looks great, thanks! I'll merge it and then add some checks to Travis to ensure CI checks for 2+3 compatibility as well for future commits.

I completely agree with you regarding enums and yield from, I just used them in the python3 branch to try these new Python 3 features. I like playing with new features on projects such as this, since it's already something useful, but still small enough to make such changes in a short time. 😃

[dnet]

The test suite held some surprises for me, but now everything works according to Travis as well, so I released 0.3 on GitHub and PyPI