[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)
Maybe having a bunch of static factory methods would be nice for this class. For example:

```java
public static SchedulerResponse OK(final String message){
  return new SchedulerResponse (SC_OK, message);
}
```

That way, the code calling this would be a little more expressive and you can make all the constants private. Having methods allows for easier refactoring in the future.