Add jsons of artwork subjects

@richbs This is a rather large PR including processed subject jsons and the scripts that produce them. `processed` contains all the final product and `bin` contains the scripts. Images downloaded in bin are not included. Let me know if you'd like to only include parts of the PR. I can resubmit :)

#### `processed` details

- `subjects`: subjects lookup jsons. Products of `process_all.py`
- `subjects-indexed`: subjects lookup jsons with preserved relationships. Products of `process_all_indexed.py`
- `usage-example`: has one file that feeds an example web implementation of the AR artwork data and the indexed subjects data. Products of `process_AR.py`. See #24
- `simplified_artwork_data.csv`: csv derived from the root `artwork_data.csv` with only `acno` and `thumbnailURL` (not excluding ones without thumbnailURL)

#### `bin` details

- python scripts used to parse all the artwork jsons and produce subject lookup files

  - `process-all.py`: subjects lookup without preserving relationships to its parent or children`
  - `process-all-indexed`: subjects lookup with relationships to its parent to children

- bash scripts used to get thumbnail images and process them

  - `get_images.sh`
  - `process_thumbnails.sh`

- `README.md` that lays out the data structure