Features/uuid

Add Sequelize.UUID with possible default values UUIDV1 or UUIDV4. Example usage:

```
Post = sequelize.define('Post', {
    title: { type: Sequelize.STRING },
    uuid: { type: Sequelize.UUID, defaultValue: Sequelize.UUIDV4 }
});

Post.sync();

Post.create({ title: 'post1' });
```

```
Executing: CREATE TABLE IF NOT EXISTS `Posts` (`title` VARCHAR(255), `uuid` CHAR(36), `id` INTEGER PRIMARY KEY AUTOINCREMENT, `createdAt` DATETIME NOT NULL, `updatedAt` DATETIME NOT NULL);
Executing: INSERT INTO `Posts` (`title`,`uuid`,`id`,`createdAt`,`updatedAt`) VALUES ('post1','933ae658-9b07-4070-a675-7395ed3854a4',NULL,'2013-12-04 22:33:12','2013-12-04 22:33:12');
```

Shamelessly created based on whito's notes from #926

Tests go for sqlite and mysql. I do not have time to install postgres right now. Maybe someone who has postgres test environment configured could verify.