Added pingpong sample app

[hiimanget]

Added a simple ping-pong app that can be executed with some commands but it is not a p2p app, just a ping-pong code. Also, created new packages for apps.

The following images are a process of the ping-pong app.

1. Initialize a new node (id: "a", host: "localhost", port: "10000").

2. Initialize another node (id: "b", host: "localhost", port: "10001").

3. Send a ping to node "b" from "a".

4. "b" received a pong from "a".

commands

• init <id> <host> <port>: initialize node info
• ping <host> <port>: send a ping message to the host
• exit: exit