Update checks / check-runs support

[bobeagan]

There are additional methods that are available in this preview feature that are not currently supported:

https://developer.github.com/v3/checks/runs/

• List check runs for a Git reference
• List check runs in a check suite holding off on this one to be included with Check Suites instead
• Get a check run
• List check run annotations

[bobeagan]

@acrobat I left off the tests for these new methods partly because the existing tests seemed incorrect and also because I wasn't sure exactly what value tests for these GET requests provided. If you have suggestions for naming the list of check runs in a check suite, I can add that in here. Sorry for the stray commit to fix my copy/paste fail. Feel free to squash on merge.