Rename Poly1305 => RbNaCl::OneTimeAuths::Poly1305

This follows the naming convention from the C source code.

"OneTimeAuths" seems a bit silly but it is more consistent. Plus it's not a name
that end users will have to deal with directly (at least in a typical case)
Merge pull request #84 from cryptosphere/one-time-auth-naming

Rename Poly1305 => RbNaCl::OneTimeAuths::Poly1305