Atom Entity changes for Activity Service.
review Comments Set 1.
added methods in Activity for XPath related changes. These need to work
for both Activity and ActivityNode. Thus overriding in Activity.
Added Comments to the methods which need to be overriden in Activity.
Merge branch 'master' into AtomEntityChanges_AS
Merge pull request #1005 from VimalDhupar/AtomEntityAS

Atom Entity changes for Activity Service.