Initial dsc_configuration resource implementation

Work in progress @btm, @jdmundrawala, @mattstratton. This is Phase 1 of DSC integration, the `dsc_configuration` resource, which allows re-use of DSC configs specified via PowerShell. The `dsc_resource` resource will most likely arrive in a future PR.

See the RFC for details on `dsc_configuration`: https://github.com/opscode/chef-rfc/pull/29.