Support for REDIS_URL environment variable

Added in a basic `urlparse` to populate `redis_init` in `heroku.conf.py`.

I've only tested this with OpenRedis but it should work for any Redis URL per what appears to be a standard format: `redis://:<passwd>@<host>:<port>/<db>` 

It reads from the ENV `REDIS_URL`, defaulting to the existing values.

I made the Heroku configuration variable deliberately generic:

```bash
$ heroku config:set REDIS_URL=`heroku config:get OPENREDIS_URL`
```