MySQL InsertFieldValueBlock should extend AbstractSetFieldBlock

Fixes #80. I'm not sure if it should be extending squel's InsertFieldValueBlock but this at least gets this sample working:
```js
var squel = require('squel').useFlavour('mysql');
var query = squel.insert()
        .into("students")
        .setFieldsRows([
          { name: "Thomas", age: 29 },
          { name: "Jane", age: 31 }
        ]);
console.log(query->toString());
```