Add aliases for accessing attributes via [] and []=

This also fixes accessing attributes via symbol name and tests to ensure
that such does not break in the future.

lib/oga/xml/element.rb

@@ -91,6 +91,8 @@ module Oga
         found ? found.value : nil
       end
 
+      alias_method :[], :get
+
       # Adds a new attribute to the element.
       #
       # @param [Oga::XML::Attribute] attribute
@@ -113,11 +115,7 @@ module Oga
         if found
           found.value = value
         else
-          if name.include?(':')
-            ns, name = name.split(':')
-          else
-            ns = nil
-          end
+          name, ns = split_name(name)
 
           attr = Attribute.new(
             :name           => name,
@@ -129,6 +127,8 @@ module Oga
         end
       end
 
+      alias_method :[]=, :set
+
       # Removes an attribute from the element.
       #
       # @param [String] name The name (optionally including namespace prefix)