This inlcudes slight refactoring and switch to https instead of ftps

Cherry pick mpris-bugfix branch from master and fix imports in listen now.

See merge request !17

Port recent changes to OMEMO

See merge request !16

[anti_spam] v1.4.3 more in CHANGELOG

Remade for GTK3

See merge request !3

Certificate is only on python >= 3.4 verified

Episode 2: no core changes required

 - An error during the upload now no longer hangs everything (GUI changes placed
   in main thread)
 - Some formatting changes
 - Update version in manifest (I think a new minor version is in order for GTK3
   support!)
 - Add self to authors in manifest (shameless plugs, whoop whoop)
 - Remove claim of supporting older versions of gajim

 - Move thumbnailing code into separate module
 - Port GdkPixbuf thumbnailer to gtk3
 - Make GdkPixbuf thumbnailer more efficient
 - Better exception logging
 - Some code reformatting

Not perfect yet, some layout issues remain, but it works.

added ability to do periodic plugin update checks (every 24 hours) and resized window to fit omemo plugin name properly