Allow using personal access token to authenticate with github

This adds the option to use a github personal access token instead of username/password to authenticate with the github api.

Before the new test will pass, you need to generate a token and add it [here](https://github.com/willroden/TeamCity.GitHub/blob/772767a10b9ffc26b1eb6766911aaeaf2697c888/tests/src/GitHubApiTest.java#L114).