Build on Travis CI using manylinux

[c-martinez]

This PR should address previous suggestion #1. Builds on Travis seem fine (except for 2.6).

Couple of issues

• Currently not building for 2.6, because it complains gives ImportError: cannot import name SkipTest
• build-wheels.sh will need to be changed in the future:
  • It force-updates auditwheel (until the docker image updates auditwheel)
  • It clones OpenCV from my own fork -- if this is the way forward to build OpenCV wheels, I'll send a PR upstream to incorporate the few changes required.

Is this useful?

[skvark]

Appveyor build file needs minor fix, but otherwise LGTM.

I don't really know the answer to the question if this is the way forward to build OpenCV wheels. If you have a better idea, please share. As the OpenCV maintainers are a bit reluctant to include Python packaging support directly to the main repo (opencv/opencv#6516), I think this separate repository + cloning OpenCV is pretty much the only option there is.

[c-martinez]

Changed the Appveyor build (hopefully it works correctly).

I think you are right and cloning OpenCV to build the wheels might be the only option. I guess what we are missing now is registering the wheels in PyPI. Do you have any experience with it? Otherwise I guess I can figure it out.

[skvark]

I have experience running a local PyPI and I can do the uploading. However, there are still a few things to do:

• the builds (cloning) have to be done from the main OpenCV repository
• versioning:
  • do we pull only the latest tag and make builds for it or do we also offer dev builds which are essentially built always when there's a commit in the OpenCV main development branch
• OS X wheels
• Are the Linux wheels allowed in PyPI (at some point they weren't)

I will create separate issues for these.