Added editing to items.
Added export/import for documents.

TODO: add tests.
Refactored `_run*` functions into `commands.py`.
Added tests for utilities.
Added tests for main.
Fixed arguments for `doorstop import` and updated grammar.
Added more tests and refactored exception handling.
Reorganized CHANGES.
Merge pull request #67 from jacebrowning/edit-documents

Expand edit functionality to support documents.