Add links as a generic mechanism for relationships

[MartyBurns]

Atom Syndication Format (the basis of RSS feeds) (RFC4287) has a general mechanism for adding linkages to parent, child, and just arbitrary relationships between catalog records.

We should consider adopting such a more general approach, as opposed isPartOf which is unidirectional.

OData (http://www.odata.org/) and GData (http://en.wikipedia.org/wiki/GData) REST APIs are largely structured on Atom and these concepts have been well tested and used.

4.2.7. The "atom:link" Element

The "atom:link" element defines a reference from an entry or feed to
a Web resource. This specification assigns no meaning to the content
(if any) of this element.

atomLink =
element atom:link {
atomCommonAttributes,
attribute href { atomUri },
attribute rel { atomNCName | atomUri }?,
attribute type { atomMediaType }?,
attribute hreflang { atomLanguageTag }?,
attribute title { text }?,
attribute length { text }?,
undefinedContent
}