feat(API): Add subscription id and source retrieval

Subscription id is needed for applications which wish to route
messages based on subscription rather than message type. This
field is now available to user applications by the sub_id
field in the message.  Subscription based applications may also
need to have access to the message source, so a get source
function has been added.

Change-Id: I796392a6e3899e8f01a3292796e54e7abd56c32f
Signed-off-by: E. Scott Daniels <daniels@research.att.com>