Merge pull request #14 from grantjenks/master

Refactor report_to_list

* Refactor permissions check into can_change_or_view
* Make group-by on multiple display fields work
* Clean-up code around display field paths and totals
* Generate values_list in a compatible (with history) way
* Refactor sorting to use defaults rather than try/catch
* Refactor choice_lists so that missing choices don't error
* Refactor display_format with formatter function
* Apply formatting to totals and arrange columns correctly
* Simplify sort_helper based on new default-argument

These changes should be entirely backwards compatible.