Add Pod::DependencyInjection

lib/cocoapods/dependency_injection.rb

+module Pod
+
+  # Provides basic support for Dependency Injection in a class.
+  #
+  module DependencyInjection
+
+    # Declares a dependency in another class specifying a default.  The class
+    # implementing this method should should initialize the dependency
+    # accessing this property.
+    #
+    # @param  [Symbol] name
+    #         the name of the dependency.
+    #
+    # @param  [Class] default_class
+    #         the default class to use for the dependency.
+    #
+    # @return [void]
+    #
+    def dependency(name, default_class)
+      singleton_class.class_eval do
+        attr_writer name
+        define_method(name) { instance_variable_get("@#{name}") || default_class }
+      end
+    end
+
+  end
+end