Added `spy.getCalls()` to allow iteration of invocations

I've come across a couple of situations where I want to use some of the Array iteration methods to filer which calls I want to interact with, for example:

```js
// Yield all calls where the third argument was passed as `foo`.
spy.getCalls()
	.filter(function (call) {
		return call.args[2] === "foo";
	})
	.forEach(function (call) {
		call.yield();
	});
```