The whole concept of specifying a pie.values() function is
not consistent with other charts.  For all pie charts,
the developer needs to make sure to just pass in an array of key-value
pairs.  Nothing more.  The example of nvd3.org is not the proper way of
passing data into pie charts.

Removing pie.values() functionality and putting console error message.
Updated examples. For issue #98, #162, #106