Attempt to eliminate sending unnecessary notifies

This PR attempts to eliminate unnecessary notifications when hostsfile content does not change.  I made `converge_by` conditional on a number of LWRP actions by adding a `content_changed?` method to the manipulator.  Moved final hostsfile content generation from the save method to its own method(s) so the new hostsfile contents can be sha-compared to the current hostsfile contents before saving.

My ruby-fu (and chef, and rspec, etc.) is somewhat lacking, so please advise if I'm taking the wrong approach or if this PR needs tuning.  Should handle issue #55, and definitely helps in another dependent cookbook I'm using.