Support relative references

I like the concept of this package and thought that being able to resolve relative paths would be an extremely useful feature to have. The first commit adds this functionality and two new tests, and the second commit fixes some issues I found with the test script.

Currently, `./` and `../` (arbitrary depth) will properly resolve. There is more that can be done to shore up the functionality (e.g. ensuring proper handling of relative paths that point to non-existent properties, DRY-ing up the code), but I wanted to propose the pull request before investing more into it.