Add Enterprise stats features

Long time ago, in the ages of version `1.*` @joeyw decided to request some Enterprise-exclusive features in issue #211. One of these features was the [Admin Stats API](https://enterprise.github.com/help/articles/admin-stats-api). I decided to add support for this API to Octokit and did so in pull #267. After 3 months, the pull was not accepted because it was not compatible with the current version of Octokit. Here it is today, fully ported to Octokit `2.*`, as requested by @pengwynn in [his comment on the ancient pull request](https://github.com/octokit/octokit.rb/pull/267#issuecomment-24011457).

**Important note: the features added use stubs for requests (basically a fake JSON files) and don't actually connect to an Enterprise server in the specs; someone needs to test them out on a real Enterprise instance!**