Add continuous integration support

* use scikit-ci: Simpler and centralized CI configuration for
  Python extensions.
  See http://scikit-ci.readthedocs.io

* use scikit-ci-addons: set of scripts useful to help drive CI.
  See http://scikit-ci-addons.readthedocs.org/

* circleci: use manylinux dockcross images including scikit-build, cmake
  and ninja.