[wip] Avatars

Still a work in progress.

Status:

- [x] `GET /v1/avatar`
- [x] `GET /v1/avatars`
- [x] `POST /v1/avatar`
- [x] `POST /v1/avatar/upload`
- [x] image processing worker to resize images
    - [x] worker server
    - [x] computer cluster with gm
    - [x] upload to s3
- [x] put processed images on s3 public bucket
- [ ] domain for s3 public bucket
- [ ] fxa-dev integration
- [ ] docs
    - [x] docs/API.md
    - [ ] docs/DESIGN.md
- [ ] tests
    - [x] tests with img.local driver
    - [ ] tests with img.aws driver

fixes #29 
nit: it's not clear what the name "preProviders" means, consider a more descriptive name e.g. "loadProviders"