first round of review feedback, mostly about documentation.
Second round of review feedback.

Be a bit more careful about transforming no-such-method into a CallNotImplementedError.
Update the 'we see that you gave a bad type' test to also see that 'nil, err' is
not treated as a bad type, but instead just treated as the regular 'you got an error
calling this method' that we would expect.
Merge branch 'api-versioning-2-facade-registry' into api-versioning-3-use-registry.

This includes the updates from review feedback on the first merge proposal.
a bit better documentation for why we have allfacades.go
Merge branch 'api-versioning-3-use-registry' into api-versioning-4-login-describes

This brings with it the review comments from the previous two branches.
add a TODO around the BestFacadeVersion function.
improved explanation for why we are sorting.