Add photo uploader

This just adds a `Foto::PhotoUploader` class that takes a `Fog` directory object and a path to a photo and then uploads the photo to the folder using the date, md5sum, and extension metadata. Path's are constructed like this: 

```
:year/:month/:day/:md5sum.:extension
```

Here is an example fake public url for a photo:

```
https://foto-test.s3.amazonaws.com/2013/12/2/8079bd4fd6d99b2a9891763df1a4cda9.jpg
```

I also renamed `PhotoStore#directory` to `PhotoStore#folder` while working on this branch, it felt more natural.