Make Probe.disable() clear the _enabled flag, and correct the socket.io probe name in the README

[Megapixel99]

Summary

Two small fixes:

1. Probe.prototype.disable() stubbed out the metrics hooks but never cleared
   the _enabled flag those hooks are restored from, so a disabled probe could
   silently come back on.
2. The README documents the socket.io probe as socketio, but the probe
   registers itself as socket.io, so the documented enable/disable value
   never matched.

1. lib/probe.js — disable() leaves _enabled true

enable() and disable() are asymmetric:

Probe.prototype.enable = function() {
  _enabled = true;                              // sets the flag
  if (_started) { /* restore metricsProbeStart/End */ }
};

Probe.prototype.disable = function() {
                                                // never cleared the flag
  this.metricsProbeStart = function() {};
  this.metricsProbeEnd = function() {};
};