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.
`Bundler.settings.all` returns keys only:

```
Bundler.settings.all
=> ["build.mysql2", "build.nokogiri", "report_anonymized_usage", "bin_path", "gemfile"]
```