Improved Error Handling and Usability of the Flink YARN Client

This pull request is addressing the following issues:
* [FLINK-995] TaskManagers are called TaskTracker in NepheleMiniCluster
* [FLINK-968] Add a yarn session parameter for the number of task slots per task manager
* Users can now set any configuration value using `-D` flags.
* I added a RPC service between the YARN Client and the ApplicationMaster for better error reporting (and also status information to the user)
* The log view in the webinterface can now handle multiple log directories (can happen with yarn)
* The webinterface shows the number of slots in the cluster
* Some renamings in the web interface (Apache Flink instead of Flink).
* I also fixed a really annoying bug in the RPC service (it swallowed basically all exceptions during deserialization of objects send over RPC).
* I've updated the documentation of the YARN client.

The changes have been tested on Amazon EMR and the 25 nodes cluster @ TUB.