Make nixpy compatible with python3 about nixpy

[gicmo]

Pr #124 should have fixed most (all?) of this. Not sure what is missing.

[jgrewe]

isn't this issue solved? Compilation with python 3 works, tests, run...

[stoewer]

If this is the case I suggest to configure Travis (and maybe Appveyor) to run and build the tests for python 3 too and close this issue.

[stoewer]

I guess I was too fast, there is still the issue with the changes in the metaclass introduced in 984fecf . This is really hacky and can be avoided.

[mpsonntag]

I'll try to configure Travis for Python 3 and come up with a solution for the metaclass changes until Thursday. If I can't come up with a nice solution, Adrian will then take over.

[gicmo]

@mpsonntag Cool! There is one thing. Currently we are building NIX from source for nixpy. Since we probably want travis to build & test both python 2 and 3 we would need to compile NIX two times. Quite a waste of time (and processing power). It would really be great if we could somehow 'reuse' the binary that has been build by travis for NIX; that or the ppa build of NIX. Or we look into bintray, which is used by homebrew. Since we are using the hdf5 that comes with travis, the hdf5 issue we had with binaries before should not be there anymore. Hopefully...