Add RDF to the user module

This issue is part of a series of patches which came out of the RDF code sprint. See the list of all RDF patches for Drupal code. Please read State of the RDF in Drupal core after the code sprint which explains the overall approach of the RDF effort in core.

To try this patch and see some RDFa output, make sure to (1) patch and enable the RDF module in core and (2) switch to the Stark theme.

Each instance in Drupal should have a URI (Universal Resource Identifier) different from the URL of the page. This is a subtle difference which can be better explained with an example. Let's take a bundle artist, it is important to differentiate the artist himself (the person) and the page which describe the artist on a given site. Typically, the creation date of the page has nothing to do with the artist and should only refer to the page. Same goes for the user who created the page, but who didn't "create" the artist (this would not make sense, unless you actually talk about the parent of the artist!). On the other side, say a field date of birth is describing the artist, and not the webpage which cannot have a date of birth. I hope this example makes the point. Typically, URIs can be generated easily by appending #this to the URL, the exception being the comments which already have a fragment to which we can add this. This patch adds URI to the user object. It also specifies the user RDF mappings via hook_rdf_mapping().