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.
Indeed, but since this is a provider for non-Windows systems, fork should be available.  Then again, it looks like OS X doesn't support fork in ruby 1.8.7, which also doesn't offer Process::spawn().  Hm.