patch 7.4.1191

Problem:    The channel feature isn't working yet.
Solution:   Add the connect(), disconnect(), sendexpr() and sendraw()
            functions.  Add initial documentation.  Add a demo server.

runtime/tools/demoserver.py

@@ -0,0 +1,87 @@
+#!/usr/bin/python
+# Server that will accept connections from a Vim channel.
+# Run this server and then in Vim you can open the channel:
+#  :let handle = connect('localhost:8765', 'json')
+#
+# Then Vim can send requests to the server:
+#  :let response = sendexpr(handle, 'hello!')
+#
+# And you can control Vim by typing a JSON message here, e.g.:
+#   ["ex","echo 'hi there'"]
+#
+# See ":help channel-demo" in Vim.
+
+import SocketServer
+import json
+import socket
+import sys
+import threading
+
+thesocket = None
+
+class ThreadedTCPRequestHandler(SocketServer.BaseRequestHandler):
+
+    def handle(self):
+        print "=== socket opened ==="
+        global thesocket
+        thesocket = self.request
+        while True:
+            try:
+                data = self.request.recv(4096)
+            except socket.error:
+                print "=== socket error ==="
+                break
+            except IOError:
+                print "=== socket closed ==="
+                break
+            if data == '':
+                print "=== socket closed ==="
+                break
+            print "received: {}".format(data)
+            try:
+                decoded = json.loads(data)
+            except ValueError:
+                print "json decoding failed"
+                decoded = [0, '']
+
+            if decoded[1] == 'hello!':
+                response = "got it"
+            else:
+                response = "what?"
+            encoded = json.dumps([decoded[0], response])
+            print "sending {}".format(encoded)
+            self.request.sendall(encoded)
+        thesocket = None
+
+class ThreadedTCPServer(SocketServer.ThreadingMixIn, SocketServer.TCPServer):
+    pass
+
+if __name__ == "__main__":
+    HOST, PORT = "localhost", 8765
+
+    server = ThreadedTCPServer((HOST, PORT), ThreadedTCPRequestHandler)
+    ip, port = server.server_address
+
+    # Start a thread with the server -- that thread will then start one
+    # more thread for each request
+    server_thread = threading.Thread(target=server.serve_forever)
+
+    # Exit the server thread when the main thread terminates
+    server_thread.daemon = True
+    server_thread.start()
+    print "Server loop running in thread: ", server_thread.name
+
+    print "Listening on port {}".format(PORT)
+    while True:
+        typed = sys.stdin.readline()
+        if "quit" in typed:
+            print "Goodbye!"
+            break
+        if thesocket is None:
+            print "No socket yet"
+        else:
+            print "sending {}".format(typed)
+            thesocket.sendall(typed)
+
+    server.shutdown()
+    server.server_close()