Expose pack in nacl.lowlevel (resolves #214)

[CMEONE]

• Exposes pack in nacl.lowlevel in both nacl.js and nacl-fast.js
• Updates nacl.min.js and nacl-fast.min.js
• Resolves issue #214

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[dchest]

Thank you for your contribution, however there's no need for a PR for such a small change. I regenerate the minified versions myself anyway for security reasons, and adding a few exported functions is easy for me to do, once the original issue has use case that's not covered by the current API.

[CMEONE]

Alright, that makes sense. It's probably better that you generate the minified versions anyway (as you stated, for security), I understand.