New features for paths

Here are some new features to allow people to find the path to root for any given item as well as to determine if one item contains another. With some tests, even. For #84
I'm not sure that this method is necessary, especially if it is not invoked anywhere else in HGrid.. Given the more primitive method `HGrid#folderContains`, we can leave it up to the user of the library to implement these higher-level helper functions.