Don't sort keys by default

[dingens]

This is a part of or followup to #110, creating a new one to structure the discussion.

Basically my point is what @feluxe said there:

sort_keys = True is nothing I would expect from a dumper by default, especially not from a YAML dumper. A dumper should convert data from one format into another, there is no reason for it to sort the data. YAML is a human readable format and used for user configs a lot. If you run a key sort on a user config you end up with a mess. It already happened to me a couple of times working with python/pyyaml. It's annoying and it shouldn't be. Just my 2 cents. :)

I agree to that. Not sorting should be the default, in the long term at least.

In the #110 discussion, the backwards compatibility issue was mentioned a couple times, but only in the form of "it might be an issue", nobody could think of an actual case where this were a problem.
Others mentioned that the sorting was not explicitly mentioned anywhere in the docs, so users couldn't really rely on it anyways.

If backwards compatibility is really the issue that prevents this, then I would suggest to use some deprecation process (now that the sort_keys option is available, users can set it to True if they really need sorting). That process would take some time, but I think it would still be better if not sorting was the default in a couple of years than never.