Add 'tahoe invite' and 'tahoe create-node --join' commands

This opens a wormhole and sends appropriate JSON down
it to a tahoe-gui using a wormhole server running on
tahoe-lafs.org

The other end uses the 'tahoe create-node' command (with
new --join option) to read the configuration JSON from
a 'tahoe invite' command

src/allmydata/_auto_deps.py

@@ -91,6 +91,9 @@ install_requires = [
     "PyYAML >= 3.11",
 
     "six >= 1.10.0",
+
+    # for 'tahoe invite' and 'tahoe join'
+    "magic-wormhole >= 0.10.2",
 ]
 
 # Includes some indirect dependencies, but does not include allmydata.