Drop Procfile, use `npm start`
index.{example.,}.js swap
Check for tiles on disk
use process.env.HOME for default directories
`mkdirp` if directories don't exist
Save `objectId` in the tileJson metadata
Split 'check for tiles on disk' into an 'adapter'
'Stack' multiple adapters

A series of adapters can be specified. For instance, first check in a
directory to see if an image has been uploaded; if not, try to download
it.

Each adapter does a node style `fn(err, success)` callback. `err`
queues up the next adapter.

TODO: The tiling shouldn't happen in the adapter. In the case of
multiple adapters (for instance 'local directory, mediabin') shouldn't
the adapter only be responsible for grabbing an image? Then the tiling
should happen after the image location has been determined.

Unless maybe there's an adapter that finds pre-tiled images…
Readme: build instructions
Readme: mention adapters
Merge branch 'npm-start' into readme
Merge branch 'readme'
Manifest: json listing all tiled images