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.

This PR addresses ["Story: User specifies that copy operation should fail when duplicate files are present"](https://github.com/gradle/gradle/blob/master/design-docs/duplicate-entries-in-archives.md).

(I know you guys are in the middle of the 1.7 release; no worries about not reviewing this right away)