Refactor and add new 'absolute' values for parent_segment/parent_segment_id

Hey Brian,

Just had a scenario where we have two independent filtering controls on a list e.g:

`news/category/media/`
`news/letter/a`

I wanted to hide the 'other' filter control if one was selected. `{parent_segment}` was perfect for this, until I encountered pagination, when the parent segment would be shifted along one. 

So I've added 'absolute' versions of the `{parent_segment}` vars, and refactored how `{parent_segment}` was created.

Can you have a look and see if it's of any use?

Thanks,

Rob