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/v1`

* The command list is sent within the `data` field of HTTP request. The commands are separated by ‘;’ character.

* For example, users can send the request via `curl` command:

	```
curl "http://{address}:{port}/Reef/v1" --data 'echo HELLO;sleep 1;echo REEF;echo ls’ 
	```
* The response notifies how many requests the scheduler received and how many tasks are waiting on the queue.

* 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.
No it does not. But I have a question at this point.
We have already 2 duplicate ShellTask classes in the examples - hellohttp and retained_eval. And the way to define Command class differs without any reason.
I think it will be better to share those kind of classes. What do you think of extracting the classes into one package - such as `o.a.r.examples.utils`?