Fix bug in current registration of validators and address #417

[agoose77]

Alternative solution to #417 (fixes #417)
Currently, the validator registration is hardcoded to use the deprecated "id" keyword, rather than using the new "id_of" function passed to create. This means that only draft 3 and 4 are currently registered. This is not visibly a bug at present, because there is only one other schema, and the fallback mechanism selects the most recent. However, for good practice we should still register the draft 6 schema.

This PR adds a Validator.ID_OF staticmethod, and call it from the registrar.

[robherring]

Presumably @Julian will want test cases for this. You can extract the ones I wrote from #417 if you want.

[agoose77]

Hmm, do you think we need to test this? I suppose we could check all the current drafts exist in the registry.
If you have those tests already, that would be very helpful!

[Julian]

Yes, every change needs tests that cover every part of it :)

@robherring I think I prefer this way a bit, but yours has tests :), so will see if we can combine those together.

[Julian]

OK, merged! Thanks.