Support registries

[jamesarosen]

Default registry: http://ahab.io/assets/{name}/{version}

If ahab.json has

{ "name": "jquery", "version": "2.0.0" }

then we won't be able to determine the filename until we fetch the asset. We probably want to introduce an ahab.json.lock so we don't have to look up the assets in order to calculate whether files need updating in the FetchTask.