Make it possible to import "matrix" directly

This aliases "matrix.client", making both the following equivalent:

  matrix = require "matrix.client"
  matrix = require "matrix"

matrix.lua

@@ -0,0 +1,8 @@
+#! /usr/bin/env lua
+--
+-- matrix.lua
+-- Copyright (C) 2016 Adrian Perez <aperez@igalia.com>
+--
+-- Distributed under terms of the MIT license.
+--
+return require "matrix.client"