Feature/memory connector comparator

/to @bajtos 
/cc @HolaMan

This PR should supersede https://github.com/strongloop/loopback-datasource-juggler/pull/131 which has the wrong implementation. I have incorporated the tests from it.
> JS Date doesn't have built-in comparison.

What version of Node are you assuming? This works for me on v0.10.28:

```
> var d1 = new Date()
undefined
> var d2 = new Date()
undefined
> d1 < d2
true
> d2 < d1
false
> d2 - d1
3396
```

I don't mind doing a conversion to `getTime()` as long as there is a comment explaining why is that a better way.

A more important matter: is the case when `val2` is falsy covered by unit-tests?