Fix Windows build

The script parts in package.json are platform dependent and the
bash if syntax does not work on Windows.
Replace git checkout with git show in Makefile
Allow commonjs modules to use module.exports directly

When using CommonJsModuleTransformer, the module can either use the import statement, or the "old" require() function. This allows the same thing for exports. If a module doesn't use any export statements, it can still export values the pre-ES6 way by assigning to exports.foo or module.exports. Traceur users can migrate to ES6 export statements gradually instead of all at once.

Fixes #598.

Review URL: https://github.com/google/traceur-compiler/pull/629

Closes #629, closes #598.
Handle error code, stdout and stderr for prepublish script

Review URL: https://github.com/google/traceur-compiler/pull/631

Closes #631.
Repl: Make sure we use the same loader everywhere

Move the repl demo code into a module and use its imports to import all of traceur. This ensures that the
repl uses the same version of Traceur throughout the repl demo.

Fixes #609

Review URL: https://github.com/google/traceur-compiler/pull/633

Closes #633, closes #609.