[points=3] support transaction for MySQL

[yinrong]

following README, we need:

async.waterfall([
(next) => {
  conn.beginTransaction(next) // new interface
},
(next) => {
  conn.insert('table7', {id: 100, data: 2}, next)
},
(rows, info, next) => {
  console.log(rows, info)
  conn.commit(next) // new interface
},
(next) => {
   // (a)
  console.log('success')
},
], err => { 
  console.log('fail')
})

The {id: 100, ...} will not be inserted unless (a) is reached.