Backward-chaining demo

Hi all, I added an initial demo for experimenting with backward-chaining based on an example taken from section 9.4.1 of "Artifical Intelligence - A Modern Approach" (Russell, Norvig).
I wasn't yet quite able to run it because I'm receiving the following error:
```
Warning file /home/sebastian/opencog/opencog/python/pln/examples/backward_chaining/criminal.scm ended with unterminated input begun at line 8
scm_unprotect_object called on unprotected object
```
What is this error referring to? I tried deleting all comments but I wasn't able to get it to disappear.