Reworked examples from v18. Patch is attached.

ChangeLog:

2014-05-30  Siva Chandra Reddy  <sivachandra@google.com>

        * NEWS (Python Scripting): Add entry about the new xmethods
        feature.

        doc/
        *  python.texi (Xmethods In Python, XMethod API)
        (Writing an Xmethod): New nodes.
        (Python API): New menu entries "Xmethods In Python",
        "Xmethod API", "Writing an Xmethod".