ACTION-1933: Completed relevant(), readonly(), and required() functions

All,

As discussed two weeks ago, I implemented the logic discussed when
there are no selected items:

- valid: combines with AND, default is true()
- relevant: combines with AND, default is true()
- readonly: combines with OR, default is false()
- required: combines with OR, default is false()

Here is the diff:

http://www.w3.org/MarkUp/Forms/wiki/index.php?title=XPath_Expressions_Module&diff=3823&oldid=3822

This completes ACTION-1933.

-Erik