Rename crypto_sign_publickey to crypto_sign_seed_keypair

The crypto_sign_seed_keypair function is analagous to
crypto_sign_keypair, except it generates a keypair for a seed instead of
a random keypair.

I think this name makes more sense than crypto_sign_publickey.
Merge pull request #7 from tarcieri/crypto_sign_seed_keypair

Rename crypto_sign_publickey to crypto_sign_seed_keypair