Add support for checking if the app is reloading (migrating)

Currently I'm doing something similar to the code below, to check if Meteor is Migrating.
```js
window.onbeforeunload = function() {
	isReloading = window.sessionStorage ? sessionStorage.getItem('Meteor_Reload') : false;
	if (!isReloading && Session.get('current_timer')) {
		return "You've got active timers. Do you want to close the page, and leave them running in the background?";
	}
}
```
The problem is, that this won't work for browsers not supporting sessionStorage.
With this pull request, I can now simply do something like the code below, and it should work for more than just browsers that support sessionStorage
```js
window.onbeforeunload = function() {
	if (!Meteor._reload.isReloading() && Session.get('current_timer')) {
		return "You've got active timers. Do you want to close the page, and leave them running in the background?";
	}
}
```