Add `its` example group builder

- Just an 'alias' to the existing `it`

This is to improve the readability of a spec in some cases, like:

```
describe(@"MyCustomSlider", ^{
    context(@"initial values", ^{
        its(@"minimumValue should be zero", ^{
            [[theValue([slider minimumValue]) should] beZero];
        }); 
    });
});
```