abstract instead of NotImplementedError

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Many methods use "abstract" instead of raising NotImplementError when they are 
destined to be implemented by a child class.

Fun fact, upon research.  I end up in 
http://norvig.com/python-iaq.html
finding an explanation for this.

Still, I think that, to avoid confusion, it should be fixed to either 
abstract() as in the FAQ or just RaiseNotImplementedError.

Otherwise, it will raise NameError which is confusing (and abstract could 
potentially be defined globally)

Original issue reported on code.google.com by alexhans...@gmail.com on 11 Oct 2011 at 6:13