Begin working on refactoring Go Package repo into a presenter interface.

Currently, only a GitHub presenter is implemented. But I want it to be
easy and natural to add other presenters (e.g., for code.google.com) by
implementing an interface.
Rename folder from presenters to presenter.
Wrap up refactor to presenter interface.

This restores previously working GitHub commit comparison presentation.
Clean up old code, improve interface naming.
Fix accidental replacement.

This is why renaming by text replacement is bad. Need to work harder to
have tools that allow renaming symbols directly.
Merge pull request #11 from shurcooL/refactor-into-presenter-interface

Refactor to have a general Presenter interface (for GitHub, code.google.com, etc.).