Fix mailbox names not being sent to the server quoted if needed.
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