Add ssh_authorized_key provider

This PR adds a provider for the `ssh_authorized_key` type.
Yeah, it works in the test but I can't see that it'd ever be invoked in real life.

Looking at https://github.com/puppetlabs/puppet/pull/2247, it appears the user type now parses the file and generates additional ssh_authorized_key resources with ensure => :absent, so instances isn't used by that feature.