Remove the uri attribute from Namespace.

Oga won't be handling URIs any time soon. The rationale is that they server zero
purpose when it comes to just parsing XML. Another goal of Oga is to make it
easy to modify and reserialize documents back to XML. If namespaces would also
store the URIs this would make this process more difficult.

lib/oga/xml/namespace.rb

@@ -5,17 +5,15 @@ module Oga
     # name and URI.
     #
     class Namespace
-      attr_accessor :name, :uri
+      attr_accessor :name
 
       ##
       # @param [Hash] options
       #
       # @option options [String] :name
-      # @option options [String] :uri
       #
       def initialize(options = {})
         @name = options[:name]
-        @uri  = options[:uri]
       end
 
       ##