#displayUser does not fully work for cross wiki references

To reproduce:

• in a subwiki use #displayUser macro with a main wiki user:

   {{velocity}}{{html}}#displayUser('xwiki:XWiki.SomeGlobalUser'){{/html}}{{/velocity}}

-> several bugs:

• no link
• id is displayed instead of the display full name

See http://tuska.myxwiki.org/xwiki/bin/view/Test/UserDisplay/ for an example.