modified collection.rb to parse rels and alternates

in order to parse relative URLs in rels, we need to parse the base/@href of
the page if there, or to pass the URL in, so added the relevant stuff to support
that.

spec/lib/microformats2/collection_spec.rb

@@ -180,7 +180,7 @@ describe Microformats2::Collection do
     describe "rels.html" do
       before do
         html = "spec/support/lib/microformats2/rels.html"
-        @collection = Microformats2.parse(html)
+        @collection = Microformats2.parse(html, nil)
       end
       describe "#to_json" do
         it "should match rels.js" do