Added initial draft of xf:serialize() to expression module

Initial draft of xf:serialize() appears here: 
http://www.w3.org/MarkUp/Forms/wiki/XPath_Expressions_Module#The_serialize.28.29_Function

This was a Category:XForms20 feature: 
http://www.w3.org/MarkUp/Forms/wiki/Parse_and_serialize_functions

It was suggested to make it compatible with XPath 3.0 serialize(),  The 
result is compatible, but not identical. It is intended to be parallel to 
the parse() function and parallel to XForms submission, including 
serialization to non-XML formats.  The differences are explained in an 
editor note, which will be fodder for discussion.

Cheers,
John M. Boyer, Ph.D.
Distinguished Engineer, IBM Forms and Smarter Web Applications
IBM Canada Software Lab, Victoria
E-Mail: boyerj@ca.ibm.com 

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