Issue title: Feature: TreemapLayoutManager for squarified treemap
 Issue body: With this library is possible to create a TreemapLayoutManager? To display the items as below?
![treemaps](https://cloud.githubusercontent.com/assets/6182310/4756662/32109a7c-5acf-11e4-9409-948ce240e17c.png)

This [http://www.win.tue.nl/~vanwijk/stm.pdf](paper) describes the Jarke J. van Wijk's algorithm for lay out items in a squarified treemap. [http://www.codeproject.com/Articles/7039/Squarified-Treemaps-in-XAML-C-using-Microsoft-Long]() In section 'How the layout works?' helps to understanding it.

This layout manager follows the pseudocode:
![sem titulo](https://cloud.githubusercontent.com/assets/6182310/4756792/499b32be-5ad0-11e4-88d1-7b80fb94a606.png)

 Comments: 
 Comment 0: Layout managers don't lay out its children globally (i.e. all at once) due to its view recycling nature. Based on the images, I can't see any fundamental barrier on implementing something like this. This layout is staggered both vertically and horizontally so the layout state will be non-trivial to implement but is doable.

I don't have any plans to support such layout in TwoWayView so I'm closing this ticket for now.

 Comment 1: Obrigado pela resposta :smile: Se eu conseguir prosseguir eu te retorno com os resultados.

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