Instead of each top-level IMAP folder being a FolderRoot object, then
children of that being FolderPath objects, this makes FolderRoot an
"empty" FolderPath, so that both top-level and descendant folders are
plain FolderPath objects. Aside from being more technically correct,
this means that empty namespace roots can now be used interchangably
with non-empty namespace roots (addressing issue #181), and custom
folder implementations no longer need to provide their own trivial,
custom FolderRoot.
To support this, a notion of an IMAP root and a local root have been
added from which all remote and local folder paths are now derived,
existing places that assume top-level == root have been fixed, and
unit tests have been added.
Since MailboxParameter inherited from StringParameter directly, it meant
that we could never send mailbox names as quoted strings. Also, the
modified-UTF-7 encoding used for mailbox names does not encode
atom-specials such as "\", so if a mailbox name contained one or more of
these, it would be sent to the mail server unquoted.
This removes the MailboxParameter class altogether, and does the
parameter conversion to/from appropriate StringParameter subclasses as
needed. Also adds unit tests for param conversion for ASCII,
atom-specials, and non-ASCII mailbox names.
Fixes issue #40
