Add ENV variables options

[nichochar]

This is a reproduction of #3 (unfortunately that build is broken because of old, flakey tests, and author is unresponsive.

The idea is still valid though, so I reproduced his work here.

[jparise]

Perhaps we should just add a single REDIS_URL environmental variable that is passed to Client.from_url?

https://github.com/andymccurdy/redis-py/blob/master/redis/client.py#L454

[nichochar]

We could, but in that case do you suggest changing the way we instantiate redis and use the from_url?

I was just extending our current call to be parametrizable. I can look into calling redis differently.

[jparise]

Yes, that's my suggestion. That would give us a single way for people to set all of the connection parameters (including username and password).

[nichochar]

I will merge this for now, since it gives more custom ability to tune your connection. But I added a ticket as per your suggestion to use URL and instantiate the client with a different call.