Monitor system timezone changes

[jeroenvollenbrock]

As discussed in #2 and nodejs/node#20026, It is possible to listen for system timezone changes and automatically reset the cache upon these events (although the lack of standard api's make it a bit cumbersome). This PR adds listeners for timezone change events on windows, osx and unix, as well as a method to register a hook. I'm really not sure if this should be a separate node module, a new version of your module (since node might start resetting the cache on its own in the future), a Node.js feature or maybe even a libuv handle (libuv/libuv#1176).

The following code can be used to test the response to system events:

var resetCache = require('reset-date-cache');

resetCache.registerHook(() => console.log(new Date().toString()));
console.log(new Date().toString());

//The timezone notifier is not referenced, set a dummy interval to keep running
setInterval(()=>{},1000); 

On unix the /etc/localtime, /etc/timezone and /etc/TZ files are monitored (this apparently has the broadest support), and on windows the HKLM\SYSTEM\CurrentControlSet\Control\TimeZoneInformation registry key is monitored. The monitoring happens using inotify, fsevents and RegNotifyChangeKeyValue, so there are no active or resource-intensive tasks continuously running.

Tested on windows, osx and linux :)