#1 Add support for the 'undefined' value

[David Holroyd]

SWFActionCodes does not know that push-type-3 is for
placing the undefined value on the stack, and
SWFActions and its implementors lack the appropriate
pushUndef() method.

This patch implements support for the 'undefined' constant.

(I worked this out by comparing the output of flasm
with the output from javaswf, for a problem movie.)

[David Holroyd]