Move `module.exports` code out of harmony package

Instead of using a small bit of code for `module.exports`, I decided to write
a comprehensive package that allowed the use of both `module.exports` and the
`exports` alias, as well as using modern methods instead of deprecated ones.

I just wanted to run this by you before merging, please check out the [before](https://github.com/mquandalle/meteor-harmony/blob/master/lib/moduleExports.js) and [after](https://github.com/christianbundy/meteor-exports/blob/master/exports.js) if you'd like.