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)
-Moved bikes to User model instead of profile
-User can copy user log to a bike log with new actsasloggable
-Added user stats
-some css was messing up checkboxes, need to figure out which stylesheet was doing that