GRADLE-2171: Added fail and warn strategies to DuplicatesStrategy.

- WARN will log a warning message for each duplicate file that is encountered.
- FAIL will throw a DuplicateFileCopyingException.
- Updated documentation and unit tests to reflect these new behaviors.
Merge pull request #174 from kylewm/DuplicatesWarnFail

GRADLE-2171: Added fail and warn strategies to DuplicatesStrategy.