d3.{unique,distinct} method(s)

When dealing with ordinal data, it would be nice to have a `d3.unique(array[, accessor])` method that returns the set of unique elements within the specified array. I'm not sure if the optional `accessor` function is necessary, but I could imagine this being useful if you wanted to find the set of unique objects that had the same attribute value.

This is somewhat similar to [`d3.nest()`](https://github.com/mbostock/d3/wiki/Arrays#wiki-d3_nest). 

Thoughts?