Prepare for publishing.

* Move `bower install` into `prepublish`.  This prevents consumers of the new published `ember-data` package from having `bower` installed globally. Also, nothing changes as far as setup requirements: running `npm install` in the root of the repo runs the `prepublish` script.
* Only publish the specific files that we want (leaves ability in the future to publish other types of things).
Looks like it was like that before. https://github.com/emberjs/data/blob/b819efbf5fecfc6c0145d6b0682cf7b283fc4482/package.json#L10

Guess it doesn't matter..