Add word-sequence numbering to relex

This is needed so that the order of the words in a sentence can be more easily deduced; used by Hujie forthe GSOC 2014 anaphora resolution project.
Oops, it would be better to modify parse-get-words.   The words in a sentence are all linked as:
```
(WordInstanceLink (stv 1.0 1.0)
   (WordInstanceNode "feet@bf71826c-487e-42df-a941-0ecd3c942a76")
   (ParseNode "sentence@2d19c7e7-2e02-4d5e-9cbe-6772174f3f4d_parse_0")
)
```
so something like 
```
(define (parse-get-words parse-node)
    (cog-chase-link 'WordInstanceLink 'WordInstanceNode parse-node))
```

should work.