[FEATURE] Added image field

This PR:

- Adds an ImageURL field to the IndexMetadata and Document.
- Adds a TestAdapter
- Addsd get/set/has methods to the Document class

BC: NO
`$sm->searchFor('foobar')->inIndex('bar')->inLocale('de')->go();`
or
`$sm->createSearchQueryBuilder('foobar')->setIndex('bar')->setLocale('de')->getResults();`

Is really a matter of choice. Currently we have almost no fluent interfaces.