Create a "Comparable" interface

This code:

``` javascript
/** @flow */

class Foo {
val: T;

lessThan(other: Foo) {
return this.val < other.val; // XXX flow doesn't like this comparison
}
}
```

produces the following error:

```
baderror.js:3:15,20: number
This type is incompatible with
baderror.js:7:12,31: string

baderror.js:3:22,27: string
This type is incompatible with
baderror.js:7:12,31: number

Found 2 errors
```

In most type systems, the `T` in `other: Foo` would be the same `T` as for the current class. This would result in number↔number and string↔string comparisons, but never number↔string.

Removing the `` in `other: Foo` makes the error go away, but now I have no confidence that Flow is actually checking anything.