Added Functionality to delete Entities

### deleteEntity()
Added 
```php
Aura\Marshal\Type\GenericType->deleteEntity($identity_value)
```
The Function takes the identity_value of the Entity to be deleted as parameter and deletes it from all indicies.

### getDeletedEntitiesIds()
Added
```php
Aura\Marshal\Type\GenericType->getDeletedEntitiesIds()
```
The function takes no arguments and returns an array of identity_values of Entities deleted using deleteEntity()

### Tests
Added several test for the Deletion Functionality