Support relative references

I like the feature set 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 the second commit fixes some issues I found with the test script.

Regardless of whether this feature ends up being included, I recommend at the very least cherry-picking 6967eec as it contains useful assertion updates to all tests and fixes a few breaking tests as well as the README section that deals with running said tests.