Added ancestors() method
Moved and refactored breadcrumbs() method
Removed breadcrumbs() and _inversedBreadcrumbs() from Page
Simplified Node's level() method using ancestors()
Updated archive plugin to use improved Node methods

The url() method has been replaced with a path() method that overrides
the default one, defined by the Node class.

Node's url() method will automatically use PageArchive's path() method
instead of the default one. This way, we don't have to repeat the url()
logic that has already been defined by Node.

Also, a bug is fixed with the URLs for day/month/year indexes. This bug
resulted in the url() method returning something like
'http://example.com/articles/2013/10/07/monthly-archive.html', instead
of 'http://example.com/articles/2013/10.html'.