Added .ifthen (conditional then)

Quite often I find myself in a situation where I want to skip a .then step when a certain condition is met. I'll be forced to write code like this:

```js
promise.then(function(value) {
    if (condition) {
        return doSomething(value);
    }
    return value;
});
```

That's quite a lot of code for something simple. I added a simple `ifthen` utility that would allow the above to be rewritten like this:

```js
promise.ifthen(condition, doSomething);
```

Much clearer, in my opinion. I added some tests for it as well.

I'm curious if others would find this useful.