support koa2 middleware

[einsitang]

just add "koa" to jwtRedisSession with async and await
use like

var JWTRedisSession = require("jwt-redis-session"),
	Koa = require("koa"),
	redis = require("redis");

var redisClient = redis.createClient(),
	secret = generateSecretKeySomehow(),
	app = Koa();

app.use(JWTRedisSession({
	client: redisClient,
	secret: secret,
	keyspace: "sess:", 
	maxAge: 86400,
	algorithm: "HS256",
	requestKey: "jwtSession",
	requestArg: "jwtToken"
}).koa);