commonjs exports fix, dynamic register adjustments

This adjusts the execute form for dynamic System.register to take argument - `require, exports, module`.

The `module` object is carefully designed to support setting via `module.exports = ...` to allow dynamic exports changing in CommonJS. This object could also be decorated with other properties by a CommonJS compatibility layer if desired allowing it to be the same as the CommonJS `module` object effectively.

The associated pipeline changes are made to support this form.

Fixes https://github.com/systemjs/systemjs/issues/136