SSO for Github Enterprise was added at 6d63f57, but it seemed not to work with
GitHub.
The curlpits were:
* The url to get the authenticate user is /user, not /api/v3/user.
* Users should set their public email for GitHub.
Describe the detail in GitHub.md and rename the original Github.md to
GitHub-Enterprise.md.
See doc/integrations/Single-Sign-On/Github.md for the usage.
Note: I thought it should work also with Github.com but it failed with "Bad
token type" error.
