variadic _.isEqual

This is useful for comparing all the items in a collection:

```javascript
_.isEqual.apply(_, items)
```

The naming is a bit awkward. In certain contexts __equal__ sounds best:

```javascript
_.equal(a, b, c)
```

In other contexts, __isEqual__ is better:

```javascript
_(a).isEqual(b)
```

I stuck with the existing name as it's the least disruptive change.

---

Clojure's [`=`][1] raises an ArityException if passed no arguments. I asked on `#clojure` whether this is because it's nonsensical to ask whether all the elements in the empty set are equal. Apparently this is not the case: it's a valid question and the answer is yes, all the elements are equal.


[1]: http://clojuredocs.org/clojure_core/clojure.core/=