Added 'variables' branch

Hi folks,

I've made an attempt to address the 'variables anywhere' issue:

  https://ndw.github.io/specification/langspec/variables/head/xproc20/

I'll put it on this weeks agenda as well.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh
Lead Engineer
MarkLogic Corporation
Phone: +1 512 761 6676
www.marklogic.com