add tests for low-level 'nacl.c' API

It also has a test vector for test_secretbox() that I generated with a C
program linked against libnacl.a .
fix raw crypto_secretbox API
fix raw crypto_box API
fix raw crypto_sign API
test_raw.py: hush flake8
fix py3 bytes-vs-str problems
README: add Changes section to document the low-level API changes
bump version to 0.3.0 so we don't forget
Merge pull request #66 from warner/fixraw

fix argument order of low-level NaCl functions