Define how `util.relative` should work and test it thoroughly

[lydell]

Just like for util.join, we should nail down how util.relative should work:

• URLs vs paths
• Edge cases, like the empty string
• Path normalization?

Then we should write tests:

• In test-util.js
• For all code that uses util.relative

This is important, so we don’t repeat edge-case bugs like #123.

[fitzgen]

From #123 and cleaned up a bit:

The invariant in the project that util.relative is used for is that all sources stored internally are relative to the source root (for consistency and so that if the source root is modified, we don't need to do any extra work). This should hold true for all valid inputs:

var absolute = <an absolute path or url>;
var root = <a prefix of absolute>;
var relative = util.relative(root, absolute);
util.join(root, relative) == absolute;

[tromey]

I don't think there's anything left to do here.