Usage statistics report

Implementation of the story from Trello: https://trello.com/c/9VAjy4le/61-usage-statistics
If it's good, I'll send a PR to `bundler-api` implementing the endpoint.
I think you can use `Bundler.ui.ask` instead of `print` and `gets`, and then you can send input to a spec like this:

```ruby
run "bundle install" do |input|
  input.write "y"
end
expect(out).to include("Allow anonymous usage")
```

Or something like that.