allow for MySQL COMMENT field-attribute in Model attribute definition

make's it possible (when using MySQL) to define a comment property in a Model attribute's definition such that the DB Schema generated by calling the `sync()` method will include a `COMMENT`.

e.g.:

```javascript
var User = sequelize.define('User', {
        username: {type: Sequelize.STRING, comment: 'Some lovely info for my DBA'}
});

// User.sync();
``` 

which would result in [roughly] the following schema SQL:

```sql
CREATE TABLE `User` IF NOT EXISTS
   `username` VARCHAR(255) COMMENT 'Some lovely info for my DBA', 
   `id` INTEGER NOT NULL auto_increment PRIMARY KEY
;
```