update Sawyer::Serializer to be configurable to other formats (like M…

…essagePack)

lib/sawyer.rb

@@ -10,5 +10,6 @@ class Error < StandardError; end
   resource
   relation
   response
+  serializer
   agent
 ).each { |f| require File.expand_path("../sawyer/#{f}", __FILE__) }

lib/sawyer/agent.rb

@@ -11,7 +11,6 @@ class << self
 
     def self.serializer
       @serializer ||= begin
-        require File.expand_path("../serializer", __FILE__)
         require 'yajl'
         Serializer.new(Yajl)
       end

lib/sawyer/serializer.rb

@@ -6,8 +6,10 @@ class Serializer
     # Public: Wraps a serialization format for Sawyer.  Nested objects are
     # prepared for serialization (such as changing Times to ISO 8601 Strings).
     # Any serialization format that responds to #dump and #load will work.
-    def initialize(format)
+    def initialize(format, dump_method_name = nil, load_method_name = nil)
       @format = format
+      @dump = @format.method(dump_method_name || :dump)
+      @load = @format.method(load_method_name || :load)
     end
 
     # Public: Encodes an Object (usually a Hash or Array of Hashes).
@@ -16,7 +18,7 @@ def initialize(format)
     #
     # Returns an encoded String.
     def encode(data)
-      @format.dump(encode_object(data))
+      @dump.call(encode_object(data))
     end
 
     # Public: Decodes a String into an Object (usually a Hash or Array of
@@ -26,7 +28,7 @@ def encode(data)
     #
     # Returns a decoded Object.
     def decode(data)
-      decode_object(@format.load(data))
+      decode_object(@load.call(data))
     end
 
     def encode_object(data)