Add examples on how to send messages to rooms

This adds two similar examples, one using matrix.client, and the other using
matrix.api.

examples/api-send-message.lua

@@ -0,0 +1,18 @@
+#! /usr/bin/env lua
+--
+-- api-send-message.lua
+-- Copyright (C) 2016 Adrian Perez <aperez@igalia.com>
+--
+-- Distributed under terms of the MIT license.
+--
+
+if #arg ~= 4 then
+   io.stderr:write(string.format("Usage: %s <homeserver-URL> <username> <password> <room>\n", arg[0]))
+   os.exit(1)
+end
+
+local api = require "matrix" .api(arg[1])
+local response = api:login("m.login.password", { user = arg[2], password = arg[3] })
+api.token = response.access_token
+api:send_message(arg[4], io.read("*a"))
+api:logout()