node-mysql api: Pool.on('connection') 

Fix node-mysql api incompatibility: Pool.on('connection') 

Allow to listen "connection" event in pool, example:
pool.on('connection', function(connection) {
  connection.query('SET SESSION auto_increment_increment=1')
});
Merge pull request #67 from danielsantiago/patch-2

node-mysql api: Pool.on('connection')