RelEx2Logic that-rule post-processing version 2

Based on #930

Implemented the `ReferenceLink` solution for that-rule.  Changed SVP-rule to output `EvaluationLink` to fit.  However, that means adjective like "happy" could be both a `PredicateNode` (from SVP-rule) and a `ConceptNode` (from admod-rule), as resulted from different sentences.  Not sure how this can be handled.

Basically the same as described on the wiki http://wiki.opencog.org/w/Claims_and_contexts except that more are included in the `ReferenceLink`.  Namely, all indirectly related links are include, so using the example sentence from the wiki, the link

```
InheritanceLink
      ConceptNode dogs@444
      ConceptNode small@555
```
will be inside the `ReferenceLink`
```
ReferenceLink
    ReferenceNode $that-pQrs123
    AndLink
        EvaluationLink
            PredicateNode fly@666
            ListLink
                ConceptNode dogs@444
        InheritanceLink
            ConceptNode dogs@444
            ConceptNode small@555
```

Instead of `ReferenceNode`, currently using `VariableNode` as a placeholder.  This can be changed easily after `ReferenceNode` is officially created.

P.S.  Still won't solve the `that(know, be)` problem.