add R.unapply

I've wanted this confusing function several times in the last month or so. It stewed in the back of my mind while I thought of possible names. Today I decided the placeholder name, `unapply`, is in fact the right name, given the `f`/`unapply(apply(f))` equivalence.

This function *could* be used to define the identity functions discussed in #511:

```javascript
var identity1 = R.compose(R.nth(1), R.unapply(R.identity));
var identity2 = R.compose(R.nth(2), R.unapply(R.identity));

identity1('a', 'b', 'c');  // => 'b'
identity2('a', 'b', 'c');  // => 'c'
```

`R.unapply` is useful for the same reason these identity functions are useful: one can reference specific elements of the arguments object, facilitating composition. `R.unapply` is more general, though, so I'm confident other uses will emerge. Even having a function for creating arrays is useful at times:

```javascript
R.unapply(R.identity)(1, 2, 3, 4, 5);  // => [1, 2, 3, 4, 5]
```

might be clearer to specify that you will get the function back:

`@sig ([*...] -> a) -> (*... -> a)`