Remove the FUD about random nonces

Now it's consistent with the documentation for public key encryption.

The NaCL pages also say random nonces are fine:
http://nacl.cr.yp.to/stream.html
docs/secret.rst: update text about nonces
Merge pull request #60 from jvarho/docfix

Rewrites the docs on SecretBox nonces. Includes more text by @warner.

Closes #60