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.