Extension of $ref resolution mechanism

[dstachowicz]

The $ref mechanism can potentially support many use cases as referred to in issue #380

It would be nice, if the resolution mechanism could be extended, too. I'd propose that $refs can refer to:

• Absolute paths via HTTP and FILE protocol (and possibly more protocols in the future)
• Paths relative to the file/resource, where the $ref is placed (as in HTML).

This would allow for a simple interpretation of contracts when all relevant files are available locally as well as when the relevant files are provided as HTTP resources (e.g. when a non-local repository or an HTML export are involved), and simplify switching and reuse between local and distributed usage.