Mention store.normalize in the store.push docs.

The `store.push` docs were written before `store.normalize` was added but they spend most of their lines explaining the concept of "normalized data" in Ember Data. This pr adds a simple example of using `store.push` with `store.normalize` to the end of the doc block.


![screen shot 2014-11-29 at 10 22 51 pm](https://cloud.githubusercontent.com/assets/54056/5236677/6304bb0e-7816-11e4-99a7-b65e5a9ebed4.png)