resolve #3335 - add SchemaType

Write migrations like 
```php
$this->createTable('{{table}}', [
    'name' => Schema::string(64)->notNull(),
    'type' => Schema::integer()->notNull()->defaultValue(10),
    'description' => Schema::text(),
    'rule_name' => Schema::string(64),
    'data' => Schema::text(),
    'created_at' => Schema::integer(),
    'updated_at' => Schema::integer(),
]);
```
will it work for postgresql and other rdbms ?