Signal composition example

Following #7, I thought I'd propose a more compositional form to the code that imports the photos.

This PR follows the ["Indent Stream Operations Consistently"](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/4b3b50a7ef3acfd8e1949f85a59044270d80eff3/Documentation/DesignGuidelines.md#indent-stream-operations-consistently) guideline. Since you're not following this in the app, feel free to suggest how you'd like it formatted, if you decide you want this PR.

I'll give further explanation via comments on specific lines.