[#157] Support the length classes

-- Add the toc_length_classes and toc_length_classes_rules 
database table into the tomatocart.sql

-- These two tables are used to store the length classes
[#157]Support the length classes

-- Add the constants about the database tables 
for length classes module
[#157]Support the length classes

--Add default length classes configuration field in the 
configuration so as to let the store owner set the default 
Length Class for their store

--Add the length classes module in the admin panel

--Add the length classes combox in the products data panel