Implement Scheduler example

This PR addresses https://issues.apache.org/jira/browse/REEF-3

It is a basic Task Scheduler example using Reef-webserver. The application receives the task (shell command) list from user and execute the tasks in a FIFO order.

* Users can send the HTTP request to the server via URL : `http://{address}:{port}/reef-example-scheduler/v1`

* Users can send the following requests
  * `/list` lists all the tasks' status.
  * `/clear` clears all the tasks waiting on the queue. Returns how many tasks are removed.
  * `/submit?cmd=COMMAND` submits a task to execute COMMAND and return the task id.
  * `/status?id=ID` returns the status of task whose id is ID.
  * `/cancel?id=ID` cancels the task whose id is ID.

* The results of tasks are written in the log files - both in driver's and evaluators'.

* By default, the application reuses existing evaluator to run Tasks. Using `-retain false` option, the driver allocates an evaluator for each command.
I'm not sure this is strictly needed here as it adds the various status event handlers to make the status of the job queryable via HTTP. In order to make this example simple, I'd drop it despite its utility.