Made (most) operator functions variadic

I've made most of the functions in util.operators accept a variable number of arguments. So you can do things like this...

```javascript
_.add(1, 1, 1)
// => 3

_.eq(2, 2, 1)
// => false

_.gt(2, 3, 1)
// => false
```

I modeled this after Clojure Core's approach since it seems pretty elegant.

I've also expanded the test suite for util.operators a good deal, replacing some placeholder tests with real ones.