Fix for ENUM support in migration.addColumn with postgres dialect.

Incorrect sql was being generated when adding an ENUM column to an existing table using migrations: 

```javascript
module.exports = {
  up: function(migration, DataTypes, done) {
    migration.addColumn('users', 'role', {
      type: DataTypes.ENUM('user', 'staff', 'admin')
    })
    .complete(done);
  }
};
```

was generating SQL like:

```sql
ADD COLUMN ('role', ENUM('user','staff','admin');
```

now produces:

```sql
ADD COLUMN ('role', 'enum_users_role');
```

Tests included.