Add RDF to the blog 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.

This is a small patch which annotates the blog entry content type and the user blog page with some RDF.