added some integration tests around creating commit status

Fixes #490 

- [x] `Create` returns expected result
- [x] `Create`, then `GetAll` returns expected result
- [x] `Create` with pending, then `Create` with success, returns in expected order
- [x] add preview support for contexts as per [ Commit Status API changes](https://developer.github.com/changes/2014-03-27-combined-status-api/)
- [x] `Create` with pending from context A, then `Create` with pending from context B, `GetAll` returns 2 statuses