Add "send" method to Mediator, provides point to point messaging and request-response patterns

## Example usage of **Mediator#send** method 

In primary DB module:
```js
  sandbox.on('db/users/get', function (query, channel, reply) {
    PrimaryDb.query(query).toList(reply)
  })
```

In redundant DB module:
```js
  sandbox.on('db/users/get', function (query, channel, reply) {
    RedundantDb.query(query).toList(reply)
  })
```

In DB client module:
```js
  sandbox.send('db/users/get', { userId: 'some-id' }, function (err, data) {
    if (err) throw err;
    console.log('User name:' + data[0].name)
  })
```

Will be only one request to the database thru the first registered DB-module. If PrimaryDb will temporary fails, then RedundantDb will process tasks instead PrimaryDb.