expose connection states for user-land

The below code just check a connection instance is connected
```js
var conn = mongoose.connection;
if (conn._readyState === 0) {
  mongoose.connect('mongodb://localhost:27017/test');
}
```
but I prefer do this like:
```js
var conn = mongoose.connection;
if (conn._readyState === mongoose.STATES.disconnected
  mongoose.connect('mongodb://localhost:27017/test');
}
```