[WIP] HKDF - RFC5869 implementation.

This is a HMACBackend agnostic implementation of the HMAC KDF described in http://tools.ietf.org/html/rfc5869.

It is a work in progress, it still needs:

- [ ] non-vector testing
- [ ] possibly a different API (generalized KDF interface may be useful)
- [ ] so much documentation