Publish salmon endpoint URL in Webfinger XRD

Specification in Panzer draft for Salmon, paragraph 3.3 mentions:
   "The 'salmon' link relation may specify a Salmon endpoint used to notify
    a resource, typically a user account, about external mentions of that
    resource. For a user account, the salmon mention link SHOULD appear in
    the Webfinger [Webfinger] XRD document for the user account."

Source:
http://salmon-protocol.googlecode.com/svn/trunk/draft-panzer-salmon-00.html#SMLR
Merge pull request #5062 from mmn/develop

Publish salmon endpoint URL in Webfinger XRD