Field hinting

The pull request introduces a setCustomField() method on field and fulttext criteria, so that the user of the API can specify which field to query instead of the default field. An example for such a custom field can be found in the schema.xml

Additionally a CustomField criterion is introduced which allows to directly query any field in the search backend. 

I am not sure I like the addition of the two methods in the criteria value objects, but I currently can't see a better way to configure this behaviour.