2.0.0: Merge pull request #49 from djs55/pkg-config

2.0.0 (2017-06-14):

• Split into 3 opam packages
  • io-page: the common definitions
  • io-page-unix: the Unix stubs
  • io-page-xen: the Xen stubs

• Add a modern Docker-based travis configuration
• Add the ability to get a unique handle of an Io_page