[V2.0 Alhpa5] Add invoice module into the admin panel

-- Add the invoice menu to the customers menu
[#6][V2.0 Alhpa5] Add invoice module into the admin panel

-- Add the list invoice action
[#6][V2.0 Alhpa5] Add invoice module into the admin panel
-- Adjust the margin of order total section
[#1][V2.0 - Alpha5] Add actions for the orders module 
in admin panel

-- Add the feature: delete order
[#11]The sub-total isn't calculated correctly 
in the shopping cart. It is always to be zero