Add support for select classes

[atroutt]

I need to be able to check for a specific tag for an image with the java client.

https://developer.clarifai.com/guide/tag#select-classes

I think this could easily be accomplished by adding a new field (a list or array of tags) to RecognitionRequest and making sure that gets passed along in the request body under the "select_classes" parameter.

If multiple tags, they should be comma separated.