Add hashtags to events and query results

Hashtags are only available through the dedicated REST API GetHashtags.
This change make them also available in the change structure reported by
events and query results.

That way it should give a chance for change consumers to use hashtags
in a similar fashion to the topic field without much overhead.

Change-Id: I763623b7f69e5d14f287c491c34e4cc489f6e621

Documentation/json.txt

diff --git a/Documentation/json.txt b/Documentation/json.txt
index 3b8a8cb..dc82ad1 100644
--- a/Documentation/json.txt
+++ b/Documentation/json.txt
@@ -29,6 +29,8 @@
 commitMessage:: The full commit message for the change's current patch
 set.
 
+hashtags:: List of hashtags associated with this change.
+
 createdOn:: Time in seconds since the UNIX epoch when this change
 was created.