Added th and br to the list of allowable elements to mitigate errors deriving from extensiondefinition-introduction.xml and because they seem harmless enough

git-svn-id: http://gforge.hl7.org/svn/fhir/trunk/build@3568 2f0db536-2c49-4257-a3fa-e771ed206c19
Fixed missing opening <p> causing invalid xhtml

git-svn-id: http://gforge.hl7.org/svn/fhir/trunk/build@3569 2f0db536-2c49-4257-a3fa-e771ed206c19