When an invalid REPL keyword is used, we actually print `undefined` as
well in the console.

    > process.version
    'v2.3.4'
    > .invalid_repl_command
    Invalid REPL keyword
    undefined
    >

This patch prevents printing `undefined` in this case.

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

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
    >

As it is, REPL doesn't honour the line continuation feature very well.
This patch

 1. keeps track of the beginning of the string literals and if they
    don't end or current line doesn't end with line continuation, then
    error out.

 2. monitors if the line continuation character is used without the
    string literal and errors out if that happens.

In REPL, if we try to evaluate an empty line, we get `undefined`.

    > process.version
    'v2.3.4'
    >
    undefined
    >
    undefined
    >

This patch prevents `undefined` from printing if the string is empty.

    > process.version
    'v2.3.5-pre'
    >
    >
    >