Add abstract create method

Now the create method belongs to the Output API and must be inmplemented in
the concrete classes.
I also added some docu there.

lib/rubella/output/base.rb

@@ -1,13 +1,35 @@
 module Rubella
   module Output
 
+    # Output base class
+    # The output class gets the storage within the already weighted and in
+    # buckets sorted content. It creates a visual representation of the given
+    # data and stores this local.
+    #
     class Base
       attr_accessor :field_size
       attr_reader   :data
 
-      def initialize field_size = 15
+      # Constructor
+      # Gets the field size to store it local. It's the size of one value of
+      # the later created visual representation. The unit depends on the kind
+      # of representation.
+      #
+      # @param field_size int size of one value
+      # @return Rubella::Output::Base
+      def initialize field_size
         @field_size = field_size
       end
+
+      # Creates a visual representation of the data in the given storage and
+      # stores this local.
+      #
+      # @param storage Rubella::Storage
+      # @return Rubella::Output::Base
+      def create storage
+        raise NotImplementedError "Please override 'create' in your concrete "+
+          "implementation"
+      end
     end
 
   end