Implement nacl.sign.keyPair.fromSeed.

Allows deriving a new sign key pair deterministically from a 32-byte
seed. Introduces a tiny change to the low-level crypto_sign_keypair():
a new argument that indicates that sk already contains seed bytes;
however crypto_sign_keypair() interface behaves exactly the same as
before if this argument is not truthful (e.g. not set).

Closes #37.
Docs: clarify fromSecretKey.