Add a storage

This storage should contain the weighted data, before it it's used in the
output classes. So we can create a timeline.
Plus we have a nice place to store the weighted data.

lib/rubella/storage.rb

@@ -0,0 +1,30 @@
+module Rubella
+
+  # The Rubella::Storage holds the weighted data.
+  # Storages can be added to get a timeline. But the Storage has no concrete
+  # knowledge of time.
+  # The Storage can have a defined length. If new data is added, it will drop
+  # the oldest entries.
+  #
+  class Storage
+    attr_reader :data
+    attr_reader :length
+
+    def initialize data, length = nil
+      @data = data
+      self.length = length
+    end
+
+    # Defines the length of the Storage.
+    # Be careful, if your Storage has more entries, tha the new length, the
+    # oldest entries will immediately be dropped.
+    #
+    def length= length
+      @length = length
+
+      # TODO drop entries, if more than new length
+    end
+
+  end
+
+end