Added credit tracking and stats

Added credit_conversions table so conversions can change over time
Added default credit conversion fixture
Added User#transaction_logs
Added User#total_credits (earned minus spent)
Added User#total_credits_spent (through transaction_logs)
Added User#total_earned_credits (complicated query which finds the conversion rate for a log depending on its created_at date)

db/migrate/20130629152047_create_credit_conversions.rb

@@ -0,0 +1,8 @@
+class CreateCreditConversions < ActiveRecord::Migration
+  def change
+    create_table(:credit_conversions) do |t|
+      t.integer  :conversion, :default => 1
+      t.timestamps
+    end
+  end
+end