Add a SECRET_KEY_SOURCE environment variable

It was too tricky to get this app up and running, but with a
`SECRET_KEY_SOURCE` environment variable it's easier. Setting this
variable will import the secret key into the newly-created keyring.

After import the environment variable is cleared. This way the data is
not leaked.

Set this variable in the shell where you run `rails s`, but there's no
need to set it in the shell where you run `rails c`. Specifically, it
only needs to be done once per `PGPPATH`.
Restructure README to mention TOTP

This app is also an experiment in time-based one-time passwords. Mention
that in the README.
Grammar in the README

That sentence needs a linking verb.
Passphrases

Add a passphrase option as well. The passphrase is encoded using SHA256
before being sent over the wire, in an effort to reduce the need to
trust us and to make it harder to get the user's actual passphrase.

Depends on the DOMCrypt extension, which is a W3C draft.
Show the TOTP secret key in addition to QR

Sometimes you're on a flight and the QR code generator just doesn't
work.