Collections are sometimes arrays not hashes

Just started playing with this gem - thanks for putting it together. 

I noticed certain queries were breaking due to collection responses being arrays in some cases. This is correct behavior according to the [docs](http://dp.la/info/developers/codex/responses/field-reference/#docs): 

sourceResource.collection = "Array of URIs of collection or aggregation of which SourceResource is a part"

I added a collection_item class and moved the collection attributes down to that level. I also renamed "collection_hash" to "collection_response" in the document class since an array is technically not a hash. 

Thanks again.