added the ability to handle username//password for cassandra

If you add a username/password in the yaml, it can use that to authenticate. If it's not there in the yaml, it will still connect and work.
```
carbon:
  host: "0.0.0.0"
  port: 2003
  rollups:
    - period: 60480
      rollup: 10
    - period: 105120
      rollup: 600
http:
  host: "0.0.0.0"
  port: 8080
logging:
  level: info
  console: true
  files:
    - "/var/log/cyanite.log"
store:
  cluster:
    - "secasprddb01-1"
    - "secasprddb01-2"
  keyspace: 'metric'
  username: "graphite"
  password: "graphite"
```
here, I'd make it a bit more malleable since other options are also necessary:

```clojure
(-> {:contact-points cluster}
      (cond-> (and username password)
                   (assoc :credentials {:user username :password password}))
      (alia/cluster)
      (alia/connect keyspace))
```