Add ability to stop listening for changes

[brandonpayton]

Sometimes bindings outlive objects that depend on them. In those cases, we need the ability to remove receive callbacks associated with those objects.

I updated Binding.prototype.receive to return a handle with a remove method that can be called to remove the callback. This means that the get(property, callback) shorthand is somewhat broken in that get cannot return both a binding and a handle for callback removal. Because of this, I removed this shorthand from bind.js and the README. While I was in the README, I also corrected mention of binding.then to binding.receive.

What are your thoughts?

P.S. In getting to know dbind, the shorthand methods were a distraction from seeing the essential simplicity of the API. Perhaps it would be better if there was no shorthand for things like binding.get(prop).put(value).

[brandonpayton]

I wrote a test for this change, but it was added on top of work by @neonstalwart to create intern-based unit tests for dbind. That branch is here: https://github.com/brandonpayton/dbind/tree/intern-remove-handle

I intended to add more tests for StatefulBinding, StatefulPropertyBinding, etc but made little progress. Rather than spend more time discerning between intentional behavior and bugs, I punted with the intent to ask you later about those objects.

[kriszyp]

I understand the desire for simplicity but it had seemed the 2-arg get() and set() seem very convenient for the most common use cases. At least we should separate out the interface change to get() for consideration from the remove() addition, I think.

[brandonpayton]

Thanks for taking a look at this. I understand. I'll separate the changes.

[brandonpayton]

Kris, here is the isolated change for your review.