Add crypto_box_seed_keypair like crypto_sign_seed_keypair

Uses sk = seed.
Test crypto_box_seed_keypair
Merge pull request #161 from jvarho/crypto-box-seed-keypair

Add crypto_box_seed_keypair
Change crypto_box_seed_keypair to use SHA-512 of the seed

This makes it compatible with what js-nacl and racl do.
Mention crypto_box_seed_keypair
Avoid redefinition of be32enc() on NetBSD.