Bucket read keys

[sevenbitbyte]

Issues resolved:

This introduces two new bucket level files:

• read-key - Allows access to read all file content and all metadata
• meta-read-key - Only allows access to read all bucket metadata

• [x] closes #15

This depends on several upstream issues

• https://github.com/datapartyjs/gpgfs-model/pull/3
• https://github.com/datapartyjs/gpg-promised/issues/9
• https://github.com/datapartyjs/gpg-promised/issues/16
• https://github.com/datapartyjs/gpg-promised/pull/25
• https://github.com/datapartyjs/gpg-promised/pull/24