implemented keyName feature to name your keys in d3.nest. added tests.

A keyName option for d3.nest, whereby you can name the keys that come out when you call .entries(). the default key is "key". For example, 

      d3.nest()
          .key(function(d) { return d[0]; })
          .keyName('state')
          .sortKeys(d3.descending)
          .entries([['AL', 1999, 200], ['CA', 1999, 350], ['CA', 2009, 550], ['AL', 2009, 250]]);
will give you:

      [
       {state: "CA", values:  [['CA', 1999, 350], ['CA', 2009, 550]]},
       {state: "AL", values:  [['AL', 1999, 200], ['AL', 2009, 250]]}
      ]