Implemented "accessor" and "undefined" flags on tangelo.accessor return values.

Brought nodelink "up to code" with such accessors.
Eliminated _notAccessors mechanism.

Tangelo widgets now should use a _defaults field instead of options, handled in
a certain way in the constructor.  This eliminates the need to explicitly track
which options are tangelo accessors and which are not.

All tangelo widget plugins have been brought up to code.
Added a tangelo.widget() function.

This function abstracts out some pesky details of using $.tangelo.widget to form
Tangelo widgets that use Vega-style accessor functions.
Removed unncessary _package.js from widget subdirectory.