added generic string predicates for where clauses

In `where` clauses, I was after the ability to do more than equality comparisons so added generic string predicates in the case  of nil values on the where map.

The `IS NULL` case can still be done via `{"fieldname IS NULL" nil}`, for example. I think this maintains the spirit of the current functionality.

Hope this helps and sorry if I'm going about this the wrong way (pull request vs JIRA bugtracker)
Greg