Allow to use custom HttpConnector when only OAuth token is given

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