Add encrypted repo backend

[creationix]

This needs to be a layer that works for github mount mode and local repos.

It will be implemented as a fs-db repo layered on top of the existing repo with aes encryption for the blobs. The user's private key will live locally in the app. This will be used to decrypt the encrypted aes key. The private key will be stored locally encrypted with a passphrase.

[creationix]

This is now proved out in the https://github.com/creationix/test-workspace and https://github.com/creationix/workspace repos and just needs to be integrated into tedit.

Probably should be done after #20 so there is somewhere sane to integrate the UI.