Added quantity validation in CartProduct
Added quantity changing into carts/index. Updated header
Added increase, decrease, delete actions in CartsController. Updated routes
Deleted some unusable specs
Moved cart's total_price calc from helper into model
Deleted build_user_cart from ApplicationController and moved into User model. Added buy_cart! method in User
Added specs for buy_cart! testing
Added ordering of cart_products in CartsController
Added possibility to increase balance by any value
Added html id to balance and total price
Changed balance/increase from get to post. Created buyings routes resources.
Created BuyingsController with done create_from_cart action.