add R.cond and rename existing function of that name

This is a function from the Lisp world I would very much like to use in my Ramda code. It can be used similarly to `R.type` to switch based on type:

```javascript
var sumNested = R.cond(
  [R.is(Number), R.identity],
  [R.is(Array), function(xs) { return R.sum(R.map(sumNested, xs)); }],
  [R.alwaysTrue, R.alwaysZero]
);

sumNested([1, [2, [3, [4, 'foo'], true, 5]]]);  // => 15
```

It's more general than `R.type`, though, so I'm sure we'll find plenty of other uses. :)