Rename msgs to commands in Request message
Add zip stuff (from zlib) for demo

Also move message handling to a new thread. This is necessary so I can
use nacl_io.
Build zlib/zip as separate pexes

There isn't a very good separation right now. I'll fix that later.
Separate out module-specific commands/types.
Move files to 2nacl/ js/ zip/ zlib/ directories
zip demo, plus lots of other hacks.

* Add zip drag+drop demo.
  - Rename example.js -> zlib_example.js
  - Add zip_example.js
  - Add zip.js with basic implementations, using the minizip library
    from zlib contrib.
* Create macros for extracting arguments: ARG_VOIDP, ARG_UINT, ARG_INT.
* Fix (paper over) a coercion bug when finding overloads.