Add Connection.call() to call procedures and return out variables

Motivation:

- More than one statement is not allowed by default. Calling procedures and fetching output variables needs 2 calls. This avoids having 2 nested queries.

Example:

```js
var mysql = require("mysql");
var connection = mysql.createConnection("...");

connection.call("my_procedure('a', 34, @foo, @bar)", function (err, data) {
  // if no error occurs, data = { foo: ... , bar: ... }
});
```

Related: #287