Move "Contributing" section into dedicated document for better GitHub integration

See https://github.com/blog/1184-contributing-guidelines.
Move development dependencies into Gemfile
Move json dependency into test group
Add json gem for testing in JRuby in 1.8-mode
Add #team_member?

This allows checking for team membership analogous to #org_member?.
Merge pull request #206 from fphilipe/add-boolean-team-member

Add #team_member?
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
Allow endpoint to be set with options[:endpoint]

In the case of exchanging a code for an oauth token in the oauth process we
need to be able to point octokit to github.com instead of api.github.com.
Instead of jumping through hoops, this commit simplifies the process by
allowing the endpoint to be passed as an option.

Currently
  Octokit.get '/'
  => GET https://api.github.com/

This Commit
  Octokit.get '/', {:endpoint => Octokit.web_endpoint}
  => GET https://github.com/
Merge pull request #208 from pengwynn/specific_endpoints

Allow endpoint to be set with options[:endpoint]
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
Use of custom objects is not necessary here
Respect superclass's initializer arity in Error

This fixes rspec-mocks and probably other mocking libs that create instances of
error classes without arguments.

E.g. in rspec-mocks `api.stub(:foobar).and_raise(Octokit::NotFound)` would cause
an error because of the missing response argument in Octokit::Error#initializer.
Add Octokit::Client::GitHub, .github_meta endpoint

.github_meta fetches the '/meta' endpoint for information about GitHub.com

http://developer.github.com/v3/meta/