Rename external mapping plugin to be external resource links - we may want to add urls to the external resource in the future and the name reads better anyway.  Update available actions to create, delete, and get.  Don't use option hooks since a blog can be associated with more than one course and using a single option doesn't cut it - even if multiple options were used it's easier to refer back to the table.  Expose link and unlink methods through xmlrpc - the link method takes a set of groups that it adds to the blog as well.