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

reaperhulk/pynacl,xueyumusic/pynacl,hoffmabc/pynacl,scholarly/pynacl,JackWink/pynacl,reaperhulk/pynacl,alex/pynacl,pyca/pynacl,lmctv/pynacl,JackWink/pynacl,ucoin-io/cutecoin,lmctv/pynacl,alex/pynacl,lmctv/pynacl,pyca/pynacl,pyca/pynacl,scholarly/pynacl,Insoleet/cutecoin,reaperhulk/pynacl,pyca/pynacl,ucoin-io/cutecoin,hoffmabc/pynacl,lmctv/pynacl,hoffmabc/pynacl,JackWink/pynacl,alex/pynacl,pyca/pynacl,xueyumusic/pynacl,scholarly/pynacl,JackWink/pynacl,lmctv/pynacl,xueyumusic/pynacl,xueyumusic/pynacl,scholarly/pynacl,reaperhulk/pynacl,ucoin-io/cutecoin,alex/pynacl,ucoin-bot/cutecoin,reaperhulk/pynacl