histograms bins via width/center/boundary

The histogram binning algorithms have been tweaked and the controls changed to use `width/center/boundary` rather than `binwidth/origin`.  `origin` is used only internally now.
Is there a reason that the first argument of this function is called `x`?  It is a little confusing with the use of `x` and `y` as positions along the axes.  Perhaps `data` would be a better name like the first argument to `ggvis()`?