remove Users.EditKey method

User keys can no longer be updated.  Instead, they should be deleted and
re-added.
update user integration test

add tests for User.Edit and User.GetKey
add support for repository subscription methods

(I kinda hate how long some of these names are... I'd much rather just
call them "subscriptions", but the API docs is pretty consistent about
calling them "repository subscriptions", so that's what we go with.
Maybe the intend to later add other kinds of subscriptions?)