gradle: Add cleanNeeded task

The cleanNeeded task will clean the project and all projects that it
depends upon.

Fixes https://github.com/bndtools/bnd/issues/589

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
gradle: Tweak description of releaseNeeded task

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #590 from bjhargrave/cleanNeeded

gradle: Add cleanNeeded task