Script commands

I've fixed script commands and added tests. Please review.

 Is it possible to have  `ResponseError` risen, if *lua* script was failed?
 Currently, ```yield gen.Task(self.client.eva, script)``` returns `ResponseError` object. Looks like following line is responsible for this: 
https://github.com/leporo/tornado-redis/blob/master/tornadoredis/client.py#L487
There are even tests, to ensure this behavior:
https://github.com/leporo/tornado-redis/blob/d0caac2a99fe052a585a8367cc2961f5cb113c93/tornadoredis/tests/test_leaks.py#L25-L26