version source URLs

Add a helper for the resolver called 'query_asset_spec'. Use this when
computing URLs for both source and output files, falling back to the
base class implementation to support environment URL mappings. Also make
any relative URLs returned from the base implementation absolute by
rebasing them onto the request host URL in order to support the common
use case of specifying a relative base URL in the webassets configuration.
Finally, version the source URLs.