Added Enterprise stats features

Added Enterprise admin stats features as requested by @joeyw in issue #211
couple of months ago. I have written the features by following [these Enterprise docs](https://support.enterprise.github.com/entries/21188836-Admin-Stats-API). I have also written spec tests for everything I've done as well as added fixtures (also from the Enterprise documentation sample response). I have followed @joeyw's [fantastic comment](https://github.com/octokit/octokit.rb/issues/211#issuecomment-19507980) also in issue #211. Notably, someone who has access to a real Enterprise installation **must** test these new features.

As for specs, these are the results on my machine from running `bundle exec rspec-queue spec`:
```
Finished in 1.32 seconds
156 examples, 0 failures

Randomized with seed 42592
```

These are from running `bundle exec rake spec`:
```
Finished in 1.98 seconds
342 examples, 0 failures

Randomized with seed 44089

Coverage report generated for RSpec to /home/rafal/Documents/Projects/octokit.rb/coverage. 2926 / 2931 LOC (99.83%) covered.
```

This is running `bundle exec rake doc:yard` *without* my changed:
```
78.29% documented
```

...and this running the same command *with* my changes:
```
78.76% documented
```

I hope that's everything. :octocat: