repl: preventing REPL crash with inherited properties

When an inherited property is used as a REPL keyword, the REPL crashes.

    ➜  Desktop  iojs
    > process.version
    'v2.3.4'
    > .toString
    readline.js:913
            stream[ESCAPE_DECODER].next(r[i]);
                                    ^
    TypeError: Cannot read property 'call' of undefined
        at REPLServer.parseREPLKeyword (repl.js:746:15)
        at REPLServer.<anonymous> (repl.js:284:16)
        at emitOne (events.js:77:13)
        at REPLServer.emit (events.js:169:7)
        at REPLServer.Interface._onLine (readline.js:210:10)
        at REPLServer.Interface._line (readline.js:549:8)
        at REPLServer.Interface._ttyWrite (readline.js:826:14)
        at ReadStream.onkeypress (readline.js:105:10)
        at emitTwo (events.js:87:13)
        at ReadStream.emit (events.js:172:7)
    ➜  Desktop

This patch makes the internal `commands` object inherit from `null` so
that there will be no inherited properties.

    > process.version
    'v2.3.5-pre'
    > .toString
    Invalid REPL keyword
    >

PR-URL: https://github.com/nodejs/io.js/pull/2163


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>