bump gradle, slf4j & netty
.travis.yml: properly run tests, print infos
UDPListener(Test): try fixing tests, always IPv4
Add tags support

Went with a simpler design than originally planned.

We might could if performance ever turns out insufficient,
but my thinking here is that if a pipeline needs higher perf,
it'd be better to create a dedicated listener and avoid
using tags altogether.

Closes #32.