Add GitLab CI file for wheels and tests
So far only (binary) wheels on Windows for Python 2.7 32-bits are supported. No builds are performed for Linux (or MacOS) yet, or for other versions of Python.
For tests a Docker environment on Linux is used, as this will allow using that same runner to easily build linux wheels in the future (see PEP-513).
NOTE: For some reason, using a GitLab runner with version 1.8.0 does not work, whereas 1.7.1 does. The former cannot find local images, which is possibly related to some default pull policy.