Implement pluggable external reference resolver

[darrelmiller]

When $ref objects point to external files we should have a mechanism to resolve this. By default we can resolve HTTP urls and maybe relative files on the filesystem. Not sure if allowing absolute paths is a good idea.

Parser should also accept URL as the source so we can download the original file..

[mwadams]

Maybe allow a "reference paths" piece of config for "absolute" file locations to which relative paths can be resolved in addition to the current directory?