don't export Redis by default

* use EXPORT_GLOBAL to control behaviour

fix nrk#68

src/redis.lua

@@ -4,10 +4,12 @@ local redis = {
     _COPYRIGHT   = 'Copyright (C) 2009-2012 Daniele Alessandri',
 }
 
--- The following line is used for backwards compatibility in order to keep the `Redis`
--- global module name. Using `Redis` is now deprecated so you should explicitly assign
--- the module to a local variable when requiring it: `local redis = require('redis')`.
-Redis = redis
+if EXPORT_GLOBAL then
+    -- The following line is used for backwards compatibility in order to keep the `Redis`
+    -- global module name. Using `Redis` is now deprecated so you should explicitly assign
+    -- the module to a local variable when requiring it: `local redis = require('redis')`.
+    Redis = redis
+end
 
 local unpack = _G.unpack or table.unpack
 local network, request, response = {}, {}, {}