Pinning/Unpinning a Post to a Collection should replicate API's behaviour

[AngeloStavrow]

Currently, WriteFreelyClient.pinPost(postId: at position: in collectionAlias: completion:) and WriteFreelyClient.unpinPost(postId: from collectionAlias: completion:) take a single post ID String.

That's not in line with how the WriteFreely API works — both /api/collections/{COLLECTION_ALIAS/pin and /api/collections/{COLLECTION_ALIAS/unpin take an array of objects ([{id: String, position: Int}] for the former, [{id: String}] for the latter).

The client should be fixed to handle bulk pinning and unpinning, just like the API. This issue is classified a bug because it diverges from the API's behaviour.

(It may be a good idea to get #7 in place first before working on this.)