Implement conversion for geom_density2d

Corresponding chart type in Plotly is 'histogram2dcontour'.
Defaults (contour spacing) are different.
I wanted to retrieve them from ggplot2 and assign corresponding values to `xbins=list(start, end, size)` (same for `ybins`) as I once did for the `geom_histogram` conversion.  But the ggplot2 parameters which controls resolution/discretization (`n`, see http://docs.ggplot2.org/current/stat_density2d.html) seems to get lost on the way...
Users can play with this in the GUI: Traces > Range/bins
/cc @pedrodz @xsaintmleux 
I noticed them upon merging master and thought I would standardize according to https://github.com/ropensci/plotly/wiki/Development-guidelines#coding-style