Add ability to upload photos.

I'm being purposely naive in my implementation and ignoring tools like [carrierwave](https://github.com/carrierwaveuploader/carrierwave) because I want to understand how everything works.

This PR refactors the photo upload class into a new `PhotoStore::Upload` class and adds an api to `Photo` for creating a Photo with an uploaded file. It also adds an upload form to the home page.

![foto-1](https://f.cloud.github.com/assets/623/1799231/48266142-6b8e-11e3-93f4-862aac3c5c03.jpg)