Support for Comparables with `valueOf`

Consider this:

``` javascript
interface Comparable {
valueOf(): number;
}

class Comparer {

greater (first: Key, second: Key): boolean {
// flow error: Cannot be compared to Comparable
return first > second;
}
}
```

The [`valueOf()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/valueOf) method gets special treatment in Javascript and returns the primitive value for an Object type.

It can be implemented by any object so that they can be used in comparisons with the '<', '==', '===' and '>' operators. However, from the example above it appears to be the case that flow doesn't understand that the `valueOf()` method in the `Comparable` interface can be used without calling `first.valueOf() > second.valueOf()` directly.

I'd love to see this implemented in flow. It will be nice if flow throws the error only if the given item is an object type and doesn't implement `valueOf()`.