Add support for unique indexes

This implements support for unique indexes, using annotations.

Example:

```php
/*
 * @var string
 *
 * @OGM\Property
 * @OGM\Index(unique=true)
 */
protected $email;
```

For this to work, the neo4jphp module must also be updated to support unique indexes. I have submitted code for that here: https://github.com/jadell/neo4jphp/pull/66

The result is that two entities of the same class with the same `email` property value (in this example) cannot be entered into the index.