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
