Refactor add_property

Change implementation of the add_property method which allows for
dynamically adding attributes from outside the gem. We made it
simpler to construct the args needed for this method and refactored
the logic and tests.

spec/lib/microformats2/collection_spec.rb

@@ -75,11 +75,17 @@ describe Microformats2::Collection do
       end
 
       describe "Format.add_property" do
+        let(:value) { "bar" }
+        before do
+          @collection.first.add_property("p-foo", value)
+        end
+
         it "creates the attr" do
-          fake_element = OpenStruct.new(inner_text: "dork")
-          fake_text_object = Microformats2::Property::Text.new(fake_element, "p-foo")
-          @collection.first.add_property(fake_text_object)
-          @collection.first.foo.to_s.should == "dork"
+          @collection.first.foo.to_s.should == value
+        end
+
+        it "allows json output of the attribute" do
+          @collection.first.to_json.should include(value)
         end
       end
     end