Added `zucchini approve` command to use screenshots from the last run as references

I've implemented a `zucchini approve` command (maybe it should be named `zucchini promote` instead?)

    ZUCCHINI_DEVICE="My iDevice" zucchini approve path/to/features
    ZUCCHINI_DEVICE="My iDevice" zucchini approve --pending path/to/features

Please note that I'm not very competent in ruby, so please review my code carefully, I'll be happy to learn from my mistakes :)