Fix README.md
Add bats test suite

- cover nbeth with some basic tests (some of them fail!)
- add a Makefile target test that runs all tests
- mark clean as phony
Complain about empty packets
Complain about too big packets

When a packet is longer than 2048 bytes a message is printed and the
program exits. All nbeth tests pass. Unfortunately the code got a little
bit messy and it needs a cleanup.
Add Travis CI configuration

Build all tools and run the bats test suite.