Remove dependency on activesupport for singleton_class

This change can be reverted when the required Ruby version >= 1.9.

Closes #264.

lib/twitter/core_ext/kernel.rb

@@ -0,0 +1,15 @@
+module Kernel
+
+  # Returns the object's singleton class.
+  def singleton_class
+    class << self
+      self
+    end
+  end unless respond_to?(:singleton_class) # exists in 1.9.2
+
+  # class_eval on an object acts like singleton_class.class_eval.
+  def class_eval(*args, &block)
+    singleton_class.class_eval(*args, &block)
+  end
+
+end

lib/twitter/cursor.rb

@@ -1,5 +1,5 @@
-require 'active_support/core_ext/kernel/singleton_class'
 require 'twitter/base'
+require 'twitter/core_ext/kernel'
 
 module Twitter
   class Cursor < Twitter::Base
@@ -23,7 +23,7 @@ def initialize(attrs, method, klass=nil)
           item
         end
       end
-      singleton_class.class_eval do
+      class_eval do
         alias_method method.to_sym, :collection
       end
     end

twitter.gemspec

@@ -2,7 +2,6 @@
 require File.expand_path('../lib/twitter/version', __FILE__)
 
 Gem::Specification.new do |gem|
-  gem.add_dependency 'activesupport', ['>= 2.3.9', '< 4']
   gem.add_dependency 'faraday', '~> 0.8'
   gem.add_dependency 'multi_json', '~> 1.3'
   gem.add_dependency 'simple_oauth', '~> 0.1.6'