Style nits and comment fixes.
Setup doc_src for Sphinx-based documentation.
Add constants to module.

BLAKE2B_SALT_SIZE
BLAKE2B_PERSON_SIZE
BLAKE2B_MAX_KEY_SIZE
BLAKE2B_MAX_DIGEST_SIZE

BLAKE2S_SALT_SIZE
BLAKE2S_PERSON_SIZE
BLAKE2S_MAX_KEY_SIZE
BLAKE2S_MAX_DIGEST_SIZE
Add documentation sources.