Hotfix/disk content type

[jcnelson]

This patch updates the disk driver to store the content-type header contents alongside any files, written into the directory whose path is $storageRoot/.gaia-metadata/$address/$filename. This is important, because these metadata files will not be directly listable or writeable by remote clients.

This PR also adds a reader/ sub-package that implements the read endpoint for the disk driver. This is a necessary component because the read endpoint is aware of the disk driver's above scheme, and will supply the appropriate content-type header on GET.

When deploying a Gaia hub that uses the disk driver, it is recommended to run both the blockstack-gaia-hub process and the blockstack-gaia-reader process, with the blockstack-gaia-reader process running behind a "real" HTTP server like nginx. The blockstack-gaia-reader process can read the same config file as the blockstack-gaia-hub, and will honor the diskSettings.storageRootDirectory field to find the top-level disk storage directory.

[kantai]

Pushed a simple error catch to the read endpoint. I was testing whether or not you can traverse with URLs like:

http://localhost:8008/1FmbX4xLUdf28RKGz7aFajvMTCvXmAnh8M/../../baz

You cannot, but it caused the sendFile to barf an uncaught promise rejection.

[kantai]

Anyways, thanks for the changes and the fix @jcnelson, please bump the version in package.json and then feel free to merge.