Change output of `shelly backup list`

I changed header in shelly backup list output, instead of:

![Screen Shot 2013-03-21 at 10 00 15](https://f.cloud.github.com/assets/127219/284908/12bfe846-9206-11e2-95ff-eabfeeb5e66e.png)

display this:

![Screen Shot 2013-03-21 at 10 01 49](https://f.cloud.github.com/assets/127219/284909/198d9ff6-9206-11e2-8f91-576c1c8c1970.png)