Node compatibility for turtlescript.

Add package.json, mocha tests (run with 'npm test').  Ensure that we can
use standard node require() to import turtlescript modules (despite our
use of AMD module syntax internally).  Refactor the write-rust-* scripts
so that they are standard AMD modules, with very thin node wrappers in
the bin/ directory.
Make new 'top-level' module w/ top-level declarations.

This reduces code duplication by putting this list of top-level declarations
needed for compiling TurtleScript with itself in one spot.