Cannot login using signing subkey without master private key

Hi,

I'm trying to login again now that keybase is using per-device keys, and I'm seeing the following error:

```
Sorry, you have a PGP private key that Keybase doesn't have a copy of.
You need to prove you're you. Any of these will unlock this computer:

- reset your account and start fresh: https://keybase.io/#account-reset
- install GPG and put your PGP private key on this machine and try again
- go back and provision with another device or paper key
```

I have an offline master key, and use three (SEC) subkeys on my device. My assumption is that I should be able to use these to sign the device key, but keybase doesn't support that yet. Is this something you might be able to look at in future, or is my assumption wrong and I'll be required to use the master private key?

Details:

```
≡ keybase --version
keybase version 1.0.8-0
≡ gpg2 --version
gpg (GnuPG/MacGPG2) 2.0.28
libgcrypt 1.6.3
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: ~/.gnupg
Supported algorithms:
Pubkey: RSA, RSA, RSA, ELG, DSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
CAMELLIA128, CAMELLIA192, CAMELLIA256
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2
≡ gpg2 -K
/Users/vemek/.gnupg/secring.gpg
-------------------------------
sec# 3744R/4733AC75 2015-09-24 [expires: 2017-09-23]
uid Mark King
uid Mark King
uid Mark King
ssb> 2048R/3E3106C2 2015-09-24
ssb> 2048R/42445BC1 2015-09-24
ssb> 2048R/1058D21E 2015-09-24
```

Any and all help appreciated, thanks guys!

Start by reproducing the login flow with the reported GPG setup: an offline master key and three signing subkeys, using the listed keybase and gpg2 versions. Trace how device provisioning handles the missing master private key. Done means a device can be authenticated using an available signing subkey without requiring the master private key.

Written by the indexing model from the issue text.