[feature] GitHub CRUD tests

I filled in the test stubs for github_update_file, github_delete_file, and github_view_file.

This replaces [PR 1243](https://github.com/CenterForOpenScience/osf/pull/1243) in the old repo, which I am going to close now.

This closes [PR 1278](https://github.com/CenterForOpenScience/osf.io/pull/1278).
I clearly did something wrong with my push; I have that:

![mock](https://cloud.githubusercontent.com/assets/3027971/5461236/794dc288-8537-11e4-8edd-cee96d181c22.png)

I'll investigate what's going on in my branch and re-push.