Add configurable support for using a money adapter

Unfortunately `shopify-money` and `money` do not play well together. Both gems provide a `Money` class and an entry point into the gem at `lib/money.rb`. This makes it pretty difficult to include double_entry in a codebase that makes use of the `shopify-money` gem.

To work around this I've added a configuration option `money_adapter` that allows double_entry to be configured to internally delegate methods to this adapter instead of using the `money` gem directly. This allows users of double_entry to provide an adapter class that allows integration with arbitrary money backends for better interoperability with their system.

This was done by adding a `DoubleEntry::Money` class that delegates its singleton methods to the adapter.

lib/double_entry.rb

@@ -3,7 +3,6 @@
 require 'active_record/locking_extensions'
 require 'active_record/locking_extensions/log_subscriber'
 require 'active_support/all'
-require 'money'
 require 'rails/railtie'
 
 require 'double_entry/version'
@@ -16,6 +15,7 @@
 require 'double_entry/locking'
 require 'double_entry/transfer'
 require 'double_entry/validation'
+require 'double_entry/money'
 
 # Keep track of all the monies!
 #

lib/double_entry/configuration.rb

@@ -4,11 +4,19 @@ module DoubleEntry
 
   class Configuration
     attr_accessor :json_metadata
+    attr_writer :money_adapter
 
     def initialize
       @json_metadata = false
     end
 
+    def money_adapter
+      @money_adapter ||= begin
+        require 'money'
+        ::Money
+      end
+    end
+
     delegate(
       :accounts,
       :accounts=,

lib/double_entry/money.rb

@@ -0,0 +1,15 @@
+# encoding: utf-8
+module DoubleEntry
+  class Money
+    class << self
+      attr_writer :adapter
+
+      def adapter
+        @adapter ||= DoubleEntry.config.money_adapter
+      end
+
+      delegate(:new, to: :adapter)
+      delegate_missing_to(:adapter)
+    end
+  end
+end

spec/double_entry/money_spec.rb

@@ -0,0 +1,16 @@
+RSpec.describe DoubleEntry::Money do
+  it 'delegates singleton methods to the adapter' do
+    DoubleEntry::Money.adapter = Class.new do
+      def self.zero
+        0
+      end
+
+      def test
+        12345
+      end
+    end
+    expect(DoubleEntry::Money.new.test).to eq(12345)
+    expect(DoubleEntry::Money.zero).to eq(0)
+    DoubleEntry::Money.adapter = ::Money
+  end
+end

spec/support/money.rb

@@ -1 +1,2 @@
-Money.locale_backend = :i18n
+require 'money'
+::Money.locale_backend = :i18n