Major refactoring for v3.0.0

- `App.java` ain't scary no more! :trollface:
- Support defaults, UDP defaults and TCP defaults
- Move the Kafka config into Typesafe Config
- Kafka client IDs & producers for every listener
- Stats per listener

Closes #22, #24, #29.
Handle, log and count exceptions parsing multipart headers
Crude filters support

Once improved and tested, should close #23.
build.gradle: no more kafka properties

Those were moved to Typesafe Config.
build.gradle: netty upgrade