Allow to use custom HttpConnector when only OAuth token is given

[ohtake]

The old pull request is #120.

If you use GitHub.connectUsingOAuth(String oauthAccessToken) or GitHub.connectUsingOAuth(String githubServer, String oauthAccessToken), the constructor of GitHub calls #getMyself.
Because there is no way to call #setConnector before the construction, #getMyself will fail if the server is behind proxies.
This pull request allows library users to use custom HttpConnector when only OAuth token is given.

Example usage is:

GitHub gh = new GitHubBuilder()
    .withEndpoint(serverAPIUrl)
    .withOAuthToken(accessToken)
    .withConnector(new HttpConnectorWithJenkinsProxy())
    .build();

[buildhive]

Kohsuke Kawaguchi » github-api #225 SUCCESS
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