R2L: negative rule post-processing, version 1

First version of negative rule to get the discussion going.  There probably will be changes depending on the discussion.  I believe there are also changes in RelEx and LG that will affect this (see opencog/relex#117).

Given a `negativemarker`
```
EvaluationLink
   PredicateNode "negativemarker"
   ListLink
      word_instance
```
generate
```
NotLink
   AndLink
      ** links involving word_instance **
```

For example, in "I do not eat injera.", `word_instance` will be `eat@1111`, while "The cat is not small.", `word_instance` will be `small@1111`

Refer to opencog/relex#87