Add PoolCluster Feature.

It's a feature related to #506

1. support for multiple hosts connection.
2. connection group & retry & selector (round-robin, random, order)

```js
// create
var poolCluster = mysql.createPoolCluster();

poolCluster.add(config); // anonymous group
poolCluster.add('MASTER', masterConfig);
poolCluster.add('SLAVE1', slave1Config);
poolCluster.add('SLAVE2', slave2Config);

// Target Group : ALL(4 group), Selector : round-robin(default)
poolCluster.getConnection(function (err, connection) {});

// Target Group : MASTER, Selector : round-robin
poolCluster.getConnection('MASTER', function (err, connection) {});

// Target Group : SLAVE1-2(wild), Selector : order
// If can't connect to SLAVE1, return SLAVE2. (remove SLAVE1 in the cluster)
poolCluster.on('remove', function (nodeId) {
  console.log('REMOVED NODE : ' + nodeId); // nodeId = SLAVE1 
});

poolCluster.getConnection('SLAVE*', 'ORDER', function (err, connection) {});

// of namespace
poolCluster.of('*').getConnection(function (err, connection) {});

var pool = poolCluster.of('SLAVE*', 'RANDOM')
pool.getConnection(function (err, connection) {});
pool.getConnection(function (err, connection) {});

// destroy
poolCluster.end();
```