Hide by post-install/update-cmd output unless in verbose mode

Hi guys!

This follows up with #158. See the description there. This:

A) Doesn't show the output of `assets:install` and `cache:clear` when running composer install/update unless you pass a `-v` or higher

B) Outputs the command being run before running it in all cases.

![screen shot 2014-08-23 at 1 30 47 pm](https://cloud.githubusercontent.com/assets/121003/4021277/bcc24562-2aeb-11e4-8a4d-df7889802a31.png)

![screen shot 2014-08-23 at 1 30 58 pm](https://cloud.githubusercontent.com/assets/121003/4021279/c060886e-2aeb-11e4-801d-bba1f2b787e0.png)

Thanks!