Fixed logs crash when source name is too long.

If the log message source_name is longer than 8 characters you get the following error:

	/home/mheath/paas/src/cli/src/main/cf.go:126 (0x41c9cf)
		displayCrashDialog: stackTrace := "\t" + strings.Replace(string(debug.Stack()), "\n", "\n\t", -1)
	/home/mheath/paas/src/cli/src/main/cf.go:26 (0x41ccbc)
		func.001: displayCrashDialog()
	/usr/local/go/src/pkg/runtime/panic.c:248 (0x42f966)
		panic: runtime·newstackcall(d->fn, (byte*)d->args, d->siz);
	/usr/local/go/src/pkg/runtime/panic.c:482 (0x43020d)
		panicstring: runtime·panic(err);
	/usr/local/go/src/pkg/runtime/slice.c:33 (0x438155)
		makeslice: runtime·panicstring("makeslice: len out of range");
	/usr/local/go/src/pkg/strings/strings.go:425 (0x4ebd34)
		Repeat: b := make([]byte, len(s)*count)
	/home/mheath/paas/src/cli/src/cf/commands/application/helpers.go:72 (0x5b3e91)
		extractLogHeader: padding := strings.Repeat(" ", expectedHeaderLength-len(logHeader))
	/home/mheath/paas/src/cli/src/cf/commands/application/helpers.go:46 (0x5b37a2)
		logMessageOutput: logHeader, coloredLogHeader := extractLogHeader(msg)
	/home/mheath/paas/src/cli/src/cf/commands/application/logs.go:103 (0x5b5f8a)
		(*Logs).displayLogMessages: cmd.ui.Say("%s", logMessageOutput(msg))
	/home/mheath/paas/src/cli/src/cf/commands/application/logs.go:60 (0x5b5c60)
		(*Logs).Run: cmd.displayLogMessages(logChan)
	/home/mheath/paas/src/cli/src/cf/commands/runner.go:52 (0x4a72cd)
		ConcreteRunner.RunCmdByName: cmd.Run(c)
	/home/mheath/paas/src/cli/src/cf/commands/api.go:1 (0x4ac5d6)
		(*ConcreteRunner).RunCmdByName: package commands
	/home/mheath/paas/src/cli/src/cf/app/app.go:399 (0x493ffc)
		func.035: cmdRunner.RunCmdByName("logs", c)
	/home/mheath/paas/src/cli/src/github.com/codegangsta/cli/command.go:73 (0x4dae74)
		com/codegangsta/cli.Command.Run: c.Action(context)
	/home/mheath/paas/src/cli/src/github.com/codegangsta/cli/app.go:101 (0x4da0c4)
		com/codegangsta/cli.(*App).Run: return c.Run(context)
	/home/mheath/paas/src/cli/src/main/cf.go:54 (0x41c559)
		main: app.Run(os.Args)
	/usr/local/go/src/pkg/runtime/proc.c:220 (0x43173f)
		main: main·main();
	/usr/local/go/src/pkg/runtime/proc.c:1394 (0x433c60)
		goexit: runtime·goexit(void)

The long source name causes the padding length to be negative. This PR solves that problem. The output is entirely pretty though since the source name part of the output over runs the message part.