Add Linux support

This adds a basic Linux provider, as requested in [MODULES-1560](https://tickets.puppetlabs.com/browse/MODULES-1560).  It does not presently update the spec tests to handle that.  Suggestions on which tests to look at fixing are welcome.
> Someone once suggested using ruby's at_exit handler, instead of forking/spawning a child watcher process.

Now that's a good idea.  I'll run some tests.