Added support to MySQL's SET type.

Added support to MySQL SET type, so now it can be used in Schema Builder.

```php
Schema::create('users', function($table)
{
    $table->increments('id');

    $table->set('column', array('value_1', 'value_2', 'value_3'))->default('value_2');

});
```

For Postgres, SQLite and SqlServer, that do not support the SET type, I replicated the way they work with the ENUM type.