Now that I've implemented the basics here, I'm less convinced there is a need for this change, somehow. Callers can downsample before or after. Really the OOM is not in the ROC curve code, but in code that might `collect()` it for local analysis. Still, might be useful to down-sample since the ROC curve probably never needs millions of points.

This is a first pass. Since the `(score,label)` are already grouped and sorted, I think it's sufficient to just take every Nth such pair, in order to downsample by a factor of N? this is just like retaining every Nth point on the curve, which I think is the goal. All of the data is still used to build the curve of course.

What do you think about the API, and usefulness?

Author: Sean Owen <sowen@cloudera.com>

Closes #3702 from srowen/SPARK-4547 and squashes the following commits:

1d34d05 [Sean Owen] Indent and reorganize numBins scaladoc
692d825 [Sean Owen] Change handling of large numBins, make 2nd consturctor instead of optional param, style change
a03610e [Sean Owen] Add downsamplingFactor to BinaryClassificationMetrics