Add Pack, Partition, and Treemap hierarchical layouts. Closes #172

[techniq]

I also updated Tree and Cluster to only pass a single param to the child render function. Based on this:

I thought it made sense to swap data and root names, so now a layout component takes in a data prop (instead of root) and the child render function receives a root prop (instead of data, along with removing descendants, links, etc).

This also makes it more consistent with BarGroup, BarStack, Projection, etc. I can be talked out of this, but it seemed like it made sense, and since we were already making a breaking change with the number of params passed to the children function, I thought it was worth changing.

[hshoff]

This is great. Happy to merge it. Tests are failing on travis right now, so don't worry about that. I'll get that fixed soon #171.

Let's keep it root over data. My reasoning is you're getting back a hierarchy root node that has all the special methods like descendants, links that wouldn't be on a raw data object like an array of objects. Once that change is made I'll get this merged, make some new tiles, add mock data and cut a 0.0.143 release.

[hshoff]

data => root

[techniq]

Done