Make scopes more flexible.

Here is my first stab at a solution to my issue with scoping from #64.

I banged my head on this for quite a while. In theory, the channel mechanism you added a while ago would have completely sufficed to solve my problem. But there was no way to utilize it from a call to `sync_new`.

After careful consideration, I decided to first be clear about what I want to accomplish. I came up with the following goals:

* Make as few changes to the code as possible
* Stay compatible to the existing API
* Enable something like `sync_new @post, locale: :en` to solve my problem from #64
* Also, I found #51 which has similar requirements. It would be nice to solve this as well.

I think my solution fits the bill. But please review this carefully. This is the biggest change I have made so far to the code base and I am not sure if I considered all the ramifications.