New method #access_token; adapt #user for oauth_token

The #user method now takes options as a second parameter. This permits
passing nil for the user along with the oauth_token in the options hash
to retrieve user details.

The new method #access_token takes the authorization code, client id,
and client secret, and returns the associated access token. This access
token can also be retrieved using the omniauth-github gem.
Fix parameter names for doc:yard compliance
Merge remote-tracking branch 'upstream/master'
Restore #user method; #access_token uses :endpoint option
Add methods for GitHub:status api endpoints.

Methods added:

`.github_status`
`.github_status_messages`
`.github_status_last_message`

Re: #209
Add bundler development dependency
require 'octokit/version'

Because require hashes based on the string, not the expanded path of the
file, it's important that we require the version file the same way that
it will be required elsewhere to avoid constant warnings.
Merge pull request #210 from pengwynn/add-github-status-api

Add methods for GitHub:status api endpoints.
Merge pull request #207 from jbence/feature/access_token

Feature/access_token