Change domain of associatedMedia to Thing

Per https://www.w3.org/wiki/WebSchemas/associatedMediaToThing and discussion at
http://lists.w3.org/Archives/Public/public-vocabs/2014May/0204.html and
http://lists.w3.org/Archives/Public/public-vocabs/2014May/0224.html, change the
domain of associatedMedia to Thing to enable other children of Thing that do
not descend from CreativeWork to have a straightforward means of including
media objects that complement their content.

Signed-off-by: Dan Scott <dan@coffeecode.net>