More obvious package.json overrides

[tbranyen]

It was not immediately obvious to me that it was possible to override the main property inside of the package.json file. Quickly experimenting with:

{
  ...,

  "jam": {
      ...,

      "main": "client-side.js"
  }
}

worked entirely as expected. This should be documented more than passively since it's incredibly powerful to separate jam specifics.

[ryanramage]

Has this section improved?

http://jamjs.org/docs#Package_json

Would that be enough to close this issue?