Plugable JSON encoder? about channels

[andrewgodwin]

There's no need to have this pluggable separately - the channel backend already gets to dictate which serialiser it uses (it just gets passed dicts and is expected to return dicts), so to change this would just mean writing a custom backend with a new serialiser, inheriting from the main one you want to use. I'll make sure the serialise/deserialise functions are broken out into class functions that can be overridden.