support for multimedia links/attachments

The attached changes are proposed as support for attaching things like photos, videos, and other "multimedia" resources to genealogical data.

In summary, a new data type, `Link` is added with the following properties:

name  | description | data type | constraints
------|-------------|-----------|------------
href | The URI to the target of the link. | URI | REQUIRED.
rel | An identifier for the semantic meaning of the link. | URI | REQUIRED. See [RFC 5988, Section 4](http://tools.ietf.org/html/rfc5988#section-4). The relation type registry is established by [RFC 5988, Section 6.2](http://tools.ietf.org/html/rfc5988#section-6.2). The suggested relation type for multimedia attachments is "related".
type | Metadata about the available media type(s) of the resource being linked to. | string | OPTIONAL.
title | Human-readable description of the semantic meaning of the link. | string | OPTIONAL.


Comments are welcome.