plans

- Have functions on repository which give a slice of commits
  by sha or date
- Make sure that group_by preserves order.  If unable to,
  embedded date sort checks across the code that can be
  toggled.
- Consider adding a changed_methods method which looks
  at the current and last commit and gives the complexity
  delta -- needed now as we have code events for methods
  just because they are in a file.  May decide that
  it is better to just handle this with external code.
- Clean up CodeEvent
- Create a Diff class with removed_methods, changed_methods, added_methods
  working off of two histories
- for removed_methods need to, for each file in a commit see whether all
  of the methods that were there in the previous commit of the file are stil there now