Begin designing the KDF interfaces. Fixes #511
Attempt some more natural language
DOcument that verify() apis should be provided
Reword for clarity
Merge pull request #516 from alex/design-principle-verify

DOcument that verify() apis should be provided
add PBKDF2 SHA1 vectors from RFC 6070
Merge pull request #517 from reaperhulk/pbkdf2-sha1-vectors

add PBKDF2 SHA1 vectors from RFC 6070
document how to build w/ custom openssl on windows
improve previous note on windows build
Try to improve the docs
add example for windows installation
Enforce that these may only be called once
Document that this is new in 0.2
Merge pull request #518 from longaccess/master

document building with custom OpenSSL installation on windows
Merge pull request #513 from alex/kdf-interface

Begin designing the KDF interfaces. Fixes #511
hkdf
Refactor HKDF support and provide vectors for tests.
This got split into SHA1 and SHA256
Break up hkdf_derive into hkdf_extract and hkdf_expand.

Testing each individually against all the vectors and actually
asserting about the intermediate state.

hkdf_derive is now just a helper function which copes with the
default arguments.
Fix pep8.