[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

Pull Request
  #23 https://github.com/apache/incubator-reef/pull/23

Closes #23