Fix NACL_SDK_ROOT path when building ports
Add puts()
Fix bug in m.getField()
Automatically convert JavaScript string -> char*.

* Move GetArg{Voidp,Int32,Uint32,Var} to message.c
* Added Arg struct. It is owned by Command and contains var, is_handle,
  and optionally a string value.
* Changed a few calls to malloc -> calloc
* Modified zip.js to use string passing, this makes the code much nicer!
Extract data from PP_Vars in Message/Command.

It's cleaner this way.