[REEF-3]Create basic Task scheduler as an example

The current Task scheduler example only supports one evaluator
at a time. This pull request addresses this issue to run multiple
evaluators simultaneously and introduce a new REST interface to
dynamically set the maximum number of evaluator.

* The new API : `/reef-example-scheduler/v1/max-eval?num={num}`.
* Quite a lot of code rearrange and refactoring are done.

JIRA: [REEF-3](https://issues.apache.org/jira/browse/REEF-3)
* Great advice! I think using <TaskEntity> would provide richer information and make this example more meaningful. I will make a pass.
* I thought running task differs from the others. But now I realize it does not at all. I am changing that to a List.

Thanks for pointing those out.