Add Parent URL Alias Token

I thought it would be nice to have the parent item's URL alias available as a token in addition to the parent's menupath. This comes in handy especially when using pathauto on the child nodes.

So here's a patch! It checks to see if the parent node has an alias, and if it does it makes it available as a token. If not, it just defaults to an empty string like the rest of the tokens in this module.