Initial port of d3Tree

Dear Christopher,

Please find a basic port of your d3Tree implementation from d3Network.

Best,

Bryan Lewis
If the width and height need to be coalesced to an equal value then this should also be done in a resize method (see https://github.com/christophergandrud/networkD3/blob/master/inst/htmlwidgets/sankeyNetwork.js#L19-L25 for an example)