created the order and line item models
bug fix on product spec
configured line item model
added constants initializer with VAT constant
added state machine and timecop to gemfile
configured order model
make locked? mehtod public
added validation for line items to check order status
added default value to orders vat
added status_after_event public method to order
added enumerize gem
added has many association from order to StatusTransitions
added StatusTransition model