Build universal wheels

[nsoranzo]

See https://packaging.python.org/guides/distributing-packages-using-setuptools/#universal-wheels

[coveralls]

Coverage remained the same at 83.211% when pulling 71ac38b on nsoranzo:patch-1 into 0dfc970 on LibraryOfCongress:master.

[acdha]

I'm really inclined to drop Python 2.6 support since it's been deprecated by the core Python development team since 2013. I'll merge this PR but we should update the docs as well.

[johnscancella]

@acdha I have yet to be on a system that required me to use python 2.6(and you know we work on some old kernels). Any reason to not drop it now?

[nsoranzo]

@acdha Thanks for the merge!

That's needed anyway to support Python 2.7 and 3.x with the same wheel.

[acdha]

See #110 for the Python 2.6 removal.

[nsoranzo]

Also, it could be pretty cool if you could provide wheels on PyPI for the next releases!

[acdha]

Will do!

[acdha]

https://pypi.org/project/bagit/ has 1.7.0 with wheels. Since it dropped Python 2.6 support I figured it was more than a point release.

[nsoranzo]

Thanks a lot!