Add cart resource

Review original models with order relations
Apply delete-paranoid on products to avoid orphaned orders

Squashed commit of the following:

commit 45e3f6f642928d7b37040556c8b8c81c6c661bb2
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 23:59:20 2013 -0300

    Add cart resource and review other resources to consider it

commit 8e885338e4f751613f6b8e317ff8182903409ac1
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 18:42:41 2013 -0300

    Add orders model

commit ad1bfc857ddb51f3e72a7e1dc58630ef96e96b75
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 18:42:24 2013 -0300

    Add migration for orders table

commit 79ca6d61541ac6684df1c7699a876cf75027dd10
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 15:42:51 2013 -0300

    Turn product deletion paranoid (just mark as deleted but don't delete in fact)

commit b4dd663c531de7af603577d237da174d257f58b1
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 15:37:23 2013 -0300

    Add delete-paranoid gem (to avoid true deletion of products)

commit e733b91c8e96d537fca12ff857187f463cc62db2
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 15:36:13 2013 -0300

    Add model for line_items

commit b35ad38608c49b94b88399f0bc0e7fee60819660
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 15:35:27 2013 -0300

    Add migration for line_items