Introduced maven wrapper and simple travisci build script

[gbatalski]

• For simpler builds and CI scenarios it is helpful to use maven wrapper
• Initial travisci script for alternative CI pipelines, which allows contributors to test changes without on-premise infrastructure, introduced

[seelmann]

Nice to introduce the possibility to run a Travis CI build. I just wonder if we should introduce the maven wrapper. For our Jenkins pipeline (see Jenkinsfile and https://builds.apache.org/blue/organizations/jenkins/dir-ldap-api-pipeline/activity/) we now use Docker with a custom image (see https://github.com/apache/directory-buildtools/tree/master/docker/maven-build) that includes Java and Maven. I think this can easily be used for the Travis CI build too.

[gbatalski]

With maven wrapper one doesn't even need to have docker or maven installed: only the jdk. For an opensource project i think, travis ci is a very nice and simple integration solution. The maven wrapper would simplify the test environment even more. The setup i propose works just fine and is very simple (https://travis-ci.org/gbatalski/directory-ldap-api/builds/528122540) especially for occasionally contributor like me. The needless complexity of jenkins setup looks to me just not state-of-the-art.

[elecharny]

I'm willing to merge this RC, but I have one question: does it impact the existing code and build in any way ? Also we need to maintain this wrapper in the long run, that would require a minimum documentation for us to get a grip on this piece of code.