Data files: add new DATA_DIR config.

Also created this directory on the server.
Data files: exclude from git.
Data files: add stubs/examples for local data.
Data files: add script to add data-by-URL symlinks!
Data files: implement file-based data store!

And use that as the default now.
Data: implement ready, failed, progress properties!
Data: backfill `ready`, `failed`, `progress` on Zoom.it data.
Data: move image info into `dzi` container.
Data: properly separate internal from public.

And in the process, implement saving & returning full DZI info!
Data files: fix string `tileSize`, `tileOverlap`.
Merge pull request #45 from zoomhub/data-files

File-based data store & Content class overhaul