* Renamed test_identical_targets to test_identical_trees.
* Added and debugged test_file_vs_dir(). Takes care of issue #74.
* Added test_identical_files() - still need to debug.
* Moved code in get_dir_items_from_db() pertaining to files to a separate function.
* Added code to diff_targets to handle file targets and file vs. dir targets.
* Debugged test_identical_files() in test_diff.py. Resolves issue #57.
* Fixed some comments in test_diff.py.
* Added test_dissimilar_trees() to test_diff.py. Resolves issue #59.
* Added test_output_option() to test_diff.py. Resolves issue #61.
* Added test_use_root_option() to test_diff.py. Resolves issue #72.
* Added test_root_prefix_option() to test_diff.py. Resolves issue #73.
* Removed a bunch of commented code.
* Added test_invalid_target_left() to test_diff.py.
* Added test_invalid_target_right() to test_diff.py.
* Added test_invalid_target_both() to test_diff.py.