Add HKDF vectors from RFC5869.
SHA256 vectors.
DOcument the primitives used in Fernet
Merge pull request #501 from alex/document-primitives

Document primitives
Merge pull request #500 from dreid/hkdf-vectors

Add HKDF vectors from RFC5869.
add library switch for windows support

Temporarily using pragma: no cover on it until we have windows coverage
on travis. Windows builds will be done via jenkins for now