Consistently implement #to_s and #inspect and remove implicit #to_s calls

In various places, we are implicitly calling `#to_s`:

```ruby
Berkshelf.ui.info "#{cookbook} #{name} #{location}"
```

Is actually:

```ruby
Berkshelf.ui.info "#{cookbook} #{name} #{location.to_s}"
```

Overloading `#to_s` is difficult to trace. I've replaced the old `#to_s` calls with `#info` and consistently implemented both `#to_s` and `#inspect` on all Berkshelf objects.

Fixes #576