add Zookeeper::Callback::Base.create method

calls the block with the callback instance itself as the first first argument.

allows libraries to extend the callback with useful information and make it available to the
called-back proc.

lib/zookeeper/callbacks.rb

@@ -3,6 +3,22 @@ module Callbacks
   class Base
     attr_reader :proc, :completed, :context
 
+    # allows for easier construction of a user callback block that will be
+    # called with the callback object itself as an argument. 
+    #
+    # @example
+    #   
+    #   Base.create do |cb|
+    #     puts "watcher callback called with argument: #{cb.inspect}"
+    #   end
+    #
+    #   "watcher callback called with argument: #<Zookeeper::Callbacks::Base:0x1018a3958 @state=3, @type=1, ...>"
+    #
+    #
+    def self.create
+      cb_inst = new { blk.call(cb_inst) }
+    end
+
     def initialize
       @completed = false
       @proc = Proc.new do |hash|
@@ -13,7 +29,6 @@ def initialize
     end
 
     def call(*args)
-      # puts "call passed #{args.inspect}"
       @proc.call(*args)
     end