Made a new CollectionAssembler class.

The idea is that Parser will handle reading and interpreting the
stream, and CollectionAssembler will handle building the collection
based on the events it gets from the parser.

lib/sgf/parser.rb

@@ -22,6 +22,7 @@ class Parser
     def parse sgf, strict_parsing = true
       error_checker = strict_parsing ? StrictErrorChecker.new : LaxErrorChecker.new
       @sgf_stream = SgfStream.new(sgf, error_checker)
+      @assembler = CollectionAssembler.new
       @collection = Collection.new
       @current_node = @collection.root
       @branches = []
@@ -90,6 +91,11 @@ def property_token_type identity
     end
   end
 
+  class CollectionAssembler
+    def initialize
+    end
+  end
+
   class IdentityToken
     def still_inside? char, token_so_far, sgf_stream
       char != "["