Extract minim's legacy 0.6 serialiser to api-elements.js

[kylef]

The JSON06Serialiser from Minim is effectively API Elements specific, and it is currently part of the public interface of Minim. Perhaps making this legacy serialiser a separate package in api-elements.js would be wise, so it is no longer public API for Minim and a separate library we can deprecate without having to cause a major release of Minim.