Implement minimal createMailbox CREATE feature.

Since folder management is not entirely trivial given the interaction of
namespaces and delimiters, it could make sense to also support some type
of slightly higher level API as well.

This could entail having listNamespaces and listMailboxes called together
and retain their results, which starts to get tricky.  So for now I figured
I would just implement the low level CREATE binding.

(In gaia-email-libs-and-more we've got some existing logic which I've now cleaned up.  I've also added a bunch of unit tests in a pending pull request.  I can try to upstream that once we reach consensus.)