reply 502 to unsupported commands

If the related method does not exist (currently ```FtpConnection.prototype._command_COMMANDNAME```), the command is not supported. The ```NOT_SUPPORTED``` object was a distinction without difference. Not only only was it unneeded overhead, but it had also fallen out-of-sync, as it still included ```FEAT``` as an unsupported command, despite having been included in the initial commit at 8d658d9590d43ccf8d04e8637b7fed937be9c1dc. ```SYST``` appears to have been marked as unsupported at cf39462b7bdaa21a2ec4a3ae0304170997384497 .

Relevant tests have also been created.  Take note, the commands included in the test file are the only ones which we are currently able to test via ```jsftp```, as said library doesn't offer full RFC959 (or beyond) compliance (nor will it allow arbitrary commands).  I have already forked the project and begun pushing upstream on an as-needed basis, but in the interim, feel free to issue raw commands connecting via TELNET or your favorite FTP client.

Additional information on reply codes at asylumfunk#17 and of course in the RFCs.