[mms-engine] Make sure that context deactivation call completes before we exit

Otherwise there is a chance that org.ofono.ConnectionContext.SetProperty call
gets canceled before it reaches ofono and MMS context may remain active after
mms-engine exits.
Merge pull request #53 from monich/deactivate

Make sure that context deactivation call completes before we exit
Version 1.0.23
Cleaned up include path for tests

Libraries in the include path must be in the list of libraries to link with
and not the other way around.
Figured out why we need an extra reference to SoupMessage

And updated the comment.