Control directory sync and credentials with dotfiles

Experimenting with an implementation of #3. The idea I'm running with now is to have users drop a `.cloud-sync.json` file in each directory they want to sync which includes the name and parent psuedopath of the destination container, along with any other metadata we decide to track.

Similarly, credentials can be stored in a `.cloud-credentials.json` file: one at the project root holding default credentials, and others adjacent to any `.cloud-sync.json` files to override them for a specific directory. It might be a nice touch to support default pyrax or fog credentials files, too.

- [x] Scan the filesystem and identify directories containing `.cloud-sync.json` files.
- [x] Parse their contents into `SyncDescription` objects.
- [x] Load `CloudCredentials` objects from `.cloud-credentials.json` files.
- [ ] Provide a command that iterates through known `SyncDescriptions` and uploads everything from those directories to Cloud Files.