Merge pull request #1800 from opscode/sersut/chef#1634

/cc: @opscode/client-engineers 

------ Backport:

DSCL User Provider Rewrite

Conflicts:
	.gitignore
	CHANGELOG.md
	RELEASE_NOTES.md
	chef.gemspec
	chef/lib/chef/provider/user/dscl.rb
	chef/spec/unit/provider/user/dscl_spec.rb
	spec/functional/resource/user/useradd_spec.rb
	spec/unit/provider/user/useradd_spec.rb