[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
the reason its "go" is because its the shortest thing that makes sense.

````
$sm->createSearch('foobar')->index('bar')->locale('fo')->go();
$sm->createSearch('foobar')->index('bar')->locale('fo')->getResults();
$sm->createSearch('foobar')->index('bar')->locale('fo')->search();
$sm->createSearch('foobar')->index('bar')->locale('fo')->execute();
````

getResults implies that the results are already there to be gotten and also conflicts with the Doctrine name, and this really is not a doctrine QB.

I can change it to `execute` but I prefer `go`. "Search for "foobar" in index "bar" in locale "go", go!"