ember command usage

Add instructions for the usage of the main ember entrypoint command in the help command. 
This is the output:

![image](https://cloud.githubusercontent.com/assets/821095/5290379/61a65208-7b2d-11e4-94c7-613197ea481c.png)

I think `--verbose` and `--silent` are used for all commands, so I moved them to the same level as `Usage ember ...`.
If this changes are ok, I can add some tests.