Bug 902560 - Part 1: Add recursive directory delete method & tests.
Bug 902560 - Part 2: Add delete testing directories in FakeProfileTestCase.
Bug 902560 - Review comment: deleteDirectory -> deleteDirectoryRecursively.
Bug 902560 - Followup: Correct fail() comment.
Bug 902560: FakeProfileTestCase deletes testing directory if it exists. r=nalexander