Added index flags support in annotation, xml & yaml mapping drivers.

It allows specifying eg. fulltext index for MysqlPlatform (the platform already supports it - https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Platforms/MySqlPlatform.php#L714 , but it couldn't be used in annotations/xml/yaml/php schemas). And also other platforms use flags for indexes (virtual, clustered etc.).

So now flags can be used like:

```php
/**
 * @Table(...,indexes={@Index(columns={"description"},flags={"fulltext"})})
 */
class Foo
{ ... }
```

```xml
...
    <indexes>
      <index name="0" columns="description" flags="fulltext"/>
    </indexes>
...
```

```yml
Foo:
  ...
  indexes:
    -
      columns: [ description ]
      flags: [ fulltext ]
```

```php
$metadata->setPrimaryTable(array(
   ...
   'indexes' => array(
       array(
          'columns' => array('description'),
          'flags' => array('fulltext'),
       )
   ),
   ...
));
```