Introduce monkey patch for date

Allows to retrieve the begin of month for a given date

lib/planet_express/date.rb

@@ -0,0 +1,8 @@
+require 'date'
+
+# Nasty monkey patching to make date objects know about the beginning of a month
+class Date
+  def begin_of_month
+    Date.new(year, month, 1)
+  end
+end