Create a test helper to run an acceptance test against a gist

[Gaurav0]

Make it easier to write an acceptance test that "loads a gist" by mocking the github api responses.

[rwjblue]

I'd like to see us have a few more acceptance test helpers to make adding tests much easier.

Off the top of my head, I think the following would be a great start:

• addFile - Takes a type, and filename, then adds a file via the normal UI.
• deleteFile - Takes the filename, and removes it via the normal UI.
• changeDisplayedFile - Takes the filename to display and either "left" or "right", then uses the normal UI to change the file displayed in panes.
• runGist - Accepts a stubbed gist, and runs the app. (Create runGist helper; minimal gist acceptance test #94)

Many of these can be extracted from the recent acceptance tests added by @Gaurav0.