Currently the assistant sets up a shared encryption key, which is checked
into git, so anyone who gets the repository can decrypt files that are
stored encrypted on special remotes.

To support using gpg keys in the assistant, we need two things:

1. Help user set up a gpg key if they don't have one. This could be a
   special-purpose key dedicated to being used by git-annex. It might be
   nice to leave the user with a securely set up general purpose key,
   but that would certianly preclude prompting for its password in the
   webapp. Indeed, the password prompt is the main problem here.
   Best solution would be to get gpg agent working on all supported 
   platforms.
2. Help user learn the gpg keys of people they want to share their repo
   with, and give them access. If the public key was recorded in the git-annex
   branch, this could be easily determined when sharing repositories with
   friends. Or, use MonkeySphere..

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Another gpg key security thing is that currently git-annex stores
crypto creds in memory while it's running. Should use locked memory. See
<https://github.com/vincenthz/hs-securemem> and
<https://github.com/vincenthz/hs-securemem/issues/1>