Define an Identifier.system for CDC NHSN Facility IDs

[jmandel]

We need a URL, ideally assigned by CDC but we can also point to a URL in the CDC domain like https://www.cdc.gov/nhsn.

[jmandel]

Per http://www.nhsndirect.com/faq, CDC is likely assigning facility-level OIDs already; there may be an OID that means "facility ID" at the root of these.