After some discussion on asterisk-dev, it was decided that the bulk of
the logic in app_stasis actually belongs in a resource module instead
of the application module.

This patch does that, leaves the app specific stuff in app_stasis, and
fixes up everything else to be consistent with that change.

 * Renamed test_app_stasis to test_res_stasis
 * Renamed app_stasis.h to stasis_app.h
   * This is still stasis application support, even though it's no
     longer in an app_ module. The name should never have been tied to
     the type of module, anyways.
 * Now that json isn't a resource module anymore, moved the
   ast_channel_snapshot_to_json function to main/stasis_channels.c,
   where it makes more sense.

Review: https://reviewboard.asterisk.org/r/2430/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@385742 65c4cc65-6c06-0410-ace0-fbb531ad65f3