added building message to build.js

There is currently a delay of 15 seconds or so when running `bin/build.js` before the user sees a messsge:

![image](https://cloud.githubusercontent.com/assets/2526/3025072/0f4fea7c-e001-11e3-853a-70caa0d1dec3.png)

This PR introduces a simple `Building ember.js ...` message which is displayed immediately:

![image](https://cloud.githubusercontent.com/assets/2526/3025093/2ead27c2-e001-11e3-8328-c45d68f749d7.png)