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.
When a non-root user without access tries to run ```shutdown```, the command reponds with:

    shutdown: Need to be root

As such, I don't think there's anything more we'd need to do in the code to handle it.  There's no good way to test if the current user has shutdown permissions without actually running the command, unfortunately.