print_detailed() should be replaced with a string builder

[glennmatthews]

Environment

• DSync version:

Proposed Functionality

Currently the print_detailed APIs on DSync, DSyncModel, Diff, and DiffElement print to stdout when called. These should be changed/refactored so that they instead construct and return an assembled string, which the caller can then print(), log.debug(), etc. as desired.

Use Case

The current functionality is useful for debugging but not for integrated use cases where logging would be more appropriate.

[glennmatthews]

Fixed by #23.