Integration of kafka headers

[safoa]

It is in order to be able to send kafka messages with headers

[romaindequidt]

What i've done:
$ git clone https://github.com/mnogu/gatling-kafka.git
$ cd gatling-kafka
$ git remote add safoa https://github.com/safoa/gatling-kafka.git
$ git fetch --all
$ git checkout safoa/master
$ git rebase origin/master

So, I create a "0.3.1-headers" release to test the last version of gatling-kafka with @safoa PR.

It works really well.

Thanks for the job and good merge!