test server launcher

Hi,
this is a helper class able to launch a background redis server:

```
with redis.test_server.TestServer({ 'port' : 9999, }) as server: 
    server.start()
    pool = redis.ConnectionPool(**server.get_pool_args())
    connection = redis.Redis(connection_pool=pool)
    print connection.config_get()
```

After the context exit the server will be shutdown.

There is an alternative interface for non-context operations as well and a test_testserver.py test.