Add crypto_sign_publickey() API

This adds a new API crypto_sign_publickey, which works similarly to the
existing crypto_sign_keypair() API, but supports a 32-byte
user-specified seed value (k).

This API is necessary for implementing Ed25519 test vectors, for
example, since we need to pass in a known seed to ensure we're
computing the public key correctly.

The name and implementation are largely borrowed from Brian Warner's
python-ed25519 library. See:

https://github.com/warner/python-ed25519/blob/d42d4b7049baf323e7113359f4f6bf88703543bc/src/ed25519.c#L21

That said, perhaps a different name would be more descriptive, since it
still returns a keypair, not just the public key? Or perhaps that's
needless bikeshedding since this name is already in use.
Merge pull request #6 from tarcieri/crypto_sign_keypair

Add crypto_sign_publickey() API