Add s3 to user

This basically just adds a `PhotoStore` model which stores S3 creds and uses the fog gem to connect to S3. Technically a `User` can have many photo stores but for now the UI limits you to one.

![foto](https://f.cloud.github.com/assets/623/1797403/aae171a0-6ad5-11e3-9366-c89238764c37.jpg)

No tests on this yet, but I'll add integration tests for the UI (or at least the controller) and for the PhotoStore directory and connection methods before merging this PR.

/cc @johndbritton