Inherits() uses go url.ResolveReference(), fixes relative $ref

Uses golang `url.ResolveReference()` to implement `Inherits()` logic. Manual implementations of `inheritsImplHttp()` and `inheritsImplFile` no longer needed. All handled by `url.ResolveReference()` out of the box.

Simplified `parse()`

Added RFC3986 examples as tests.

https://github.com/janmentzel/gojsonschema tests still pass.