Embedding Prosody using AppImage:

Thanks to this commit, there is no more need to manually install Prosody
on the server.
The plugin now build and embed an AppImage of Prosody.

In this commit:
* building and using a Prosody AppImage.
* Adding a launcher in the AppImage: the first command argument tells if
  we want to run prosody or prosodyctl
* prosodyCtl functions now uses the AppImage.
* Prosody AppImage: extract once at the startup, then run the squashfs

prosody/launcher.lua

@@ -0,0 +1,13 @@
+#!/usr/bin/env lua
+
+-- This file is a launcher, that takes the first argument to choose what to launch.
+
+local what = table.remove(arg, 1);
+if what == 'prosody' then
+  dofile('/usr/bin/prosody');
+elseif what == 'prosodyctl' then
+  dofile('/usr/bin/prosodyctl');
+else
+  print("Unknown command: "..what);
+  os.exit(1);
+end