Mesos integration of Apache Flink

Hi,

this PR is a mesos client for Flink. It still needs to be tested which i will do as soon as I have access to a cluster. On a single machine with mesos running it works fine. Even though it is not ready to merge, I want to gather feedback and suggestions on it. 

I tried to make everything as similar to the yarn client as possible. For an idea on how this works there is a mesos_setup.md. 
The method is defined in the Scheduler Interface and is used to receive messages from the executors. It allows internal communication between the executors and the scheduler. But our project does not use it at the moment and the executors don't send any message, so it is never called. Also the documentation reveals that the messages are not very reliable.