Add continuous with docker and travis-ci

[Gjacquenot]

This PR add continuous integration with travis.

Result can be seen here

If accepted, one can add a build badge with the following link: https://travis-ci.org/Gjacquenot/matplotlib-cpp.svg?branch=master

It should be updated to
https://travis-ci.org/lava/matplotlib-cpp.svg?branch=master

[lava]

Hi, thanks for the PR. I don't mind adding this in principle, but could you move the Dockerfile, etc. into contrib/? I basically don't want to be responsible for updating/maintaining the additional infrastructure, so I think this way it would be more clear that users are on their own if they find the docker build broken some day.

[Gjacquenot]

@lava : I moved everything to the contrib directory. I even have moved the docker_build target to the contrib directory. The only file I could not move is the .travis.yml that has to be located at root directory to trigger continuous integration with https://travis-ci.org.

Is that ok with you?

[lava]

Hi, I had to drop your Makefile changes because they were conflicting with other commits in the meantime, but apart from that I just merged your PR. Thanks for contributing!