add --dependency-tree to show dependencies and versions

```
~/github/diode:-$ gp --dependency-tree                                                                                              2013-09-24 0:33:03
/// g o p a c k ///
showing dependency tree info
+- gorilla
  - github.com/gorilla/mux @ master
+- bradfitz
  - github.com/bradfitz/gomemcache @ master
```

cc @calavara