Added UNIQUE support

Added UNIQUE support, SQLite will automatically create the INDEX.

Example usage:
```javascript
var Data = schema.define('Data', {
        // num: { type: Number, primaryKey: true, autoIncrement: true},
	key: { type: String, index: true, unique: true, null: false },
	value: String
});
```

This will generate:
```SQL
CREATE TABLE `Data` (
  `id` INTEGER PRIMARY KEY,
  `key` VARCHAR(255) NOT NULL UNIQUE,
  `value` VARCHAR(255) NULL
);
```