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.
Merge pull request #43 from asutherland/create-mailbox

Implement minimal createMailbox CREATE feature.