Support relative URLs from document info plugins

PDF usages might want to specify relative URLs to get around proxy
restrictions with a custom viewer. Even link tags support relative
URLs for rel=canonical though it's probably a bad idea as a publisher.
In any case, we should clearly just do this.

h/static/scripts/annotator/guest.coffee

+baseURI = require('base-url')()
 raf = require('raf')
 Annotator = require('annotator')
 $ = Annotator.$
@@ -105,7 +106,7 @@ module.exports = class Guest extends Annotator
 
     return metadataPromise.then (metadata) =>
       return uriPromise.then (href) =>
-        uri = new URL(href)
+        uri = new URL(href, baseURI)
         uri.hash = ''
         uri = uri.toString()
         return {uri, metadata}

package.json

@@ -4,6 +4,7 @@
   "version": "0.0.0",
   "description": "The Internet, peer reviewed.",
   "dependencies": {
+    "base-url": "^1.0.0",
     "browserify": "^9.0.3",
     "browserify-shim": "^3.8.3",
     "clean-css": "2.2.2",