GitHub Enterprise Admin APIs

#211 #306

Work in progress for the GitHub Enterprise Admin APIs. Currently this pull request covers the Admin Stats and the Search Indexing APIs, tested against 11.10.327. I subclassed `Octokit::Client` to `Octokit::EnterpriseAdminClient` that includes the additional admin only methods.

I experimented with example groups in the SearchIndexing tests. I'm not sure how I feel about the duplication trade yet.

I haven't tackled the Management Console APIs yet, but I have experimented with them. I've run into a few quirks with them though. 

The [Retrieve Keys](https://enterprise.github.com/help/articles/management-console-api#retrieve-keys) endpoint returns the wrong `Content-Type`, it seems to always be `text/html` even though it is sending back JSON. 

Uploading the 500MB~ software package is a small hurdle, using the same method as the release file upload methods results in the ruby process taking several gigs of memory. Something doesn't seem right with it taking several times the file size in memory.

Seeing as these apis address such a small number of users relatively I wonder if this will get used, but a small part of me wants to make it complete, another part of me - the nerdy one just wanted an excuse to download GitHub Enterprise and try it out, lastly, seeing an issue I opened nearly a year ago is somewhat depressing to me, what say you?