Allow addons to preserve `main` entry point.

This allows packages to support both standard NPM usage (where the
`main` entry point is to the actual library) AND Ember CLI usage by
using the `ember-cli-addon-main` property if it is present.

Packages like `ic-ajax` and `emberfire` can use this to support both
types of consumers.
Merge pull request #1052 from rjackson/allow-addons-to-not-change-main

Allow addons to preserve `main` entry point.