Gitter integration into Travis CI
HipChat integration into Travis CI
Slack integration into Travis CI
Merge branch 'chat-integration-travis' into develop

I've added integration into our Travis-CI scripts to interact with
three different free chat services:

 - http://www.hipchat.com/gxkW23boK
 - https://gitter.im/matlab2tikz/matlab2tikz
 - https://matlab2tikz.slack.com

I would evaluate each of them in terms of features (e.g. mobile apps),
ease of use, availibility of public chat access (?), history retention, ...

Having such a chat room would allow us to communicate outside of GitHub
and not abuse issues for discussions. The chat form is more short-lived,
but that has the advantage that if some people are available at the same time
that we can make decisions in shorter timespans.

Even if we don't use those, such a chat room integrates both with GitHub and
Travis CI. This integration gives an overview of what has happened over time
to the repository and the tests.

Eventually, we should (however) settle for a single service to communicate.
But the best way, IMHO, is to just test-drive them and see what we like.