Allow dynamically loaded namespace and provide (depend)

This is still missing tests, the docstrings should be self-explanatory and here is an example config:

```clojure
(local-repo "/home/pyr/repo")

(logging/init {:file "riemann.log" :console true})

(instrumentation {:enabled? false})

(depend :kafka org.spootnik/riemann-kafka "0.2.1" {:offline? true})

(kafka/kafka-consumer {:topic "events"
                       :zookeeper.connect "localhost:2181"
                       :group.id "riemann"})

(periodically-expire 1)

(streams
 prn)
```