Add users and session resources

Apply authentication and authorization

Squashed commit of the following:

commit a6e964102308d4922efb9040f8c2860c1dcb707b
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 04:22:59 2013 -0300

    Add some useful links through layout just to facilitate navigation

commit f51f888d0bab47d579ddb14f5fc2e2fc711e865a
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 04:16:36 2013 -0300

     Add on admin namespace a resource for users

commit b76697f53b2a68f3f35beb657889f61a1fe0c3d9
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 03:23:05 2013 -0300

    Require admin user on admin/products actions

commit 000e9df51be815489d97e9bf918247df5c11f628
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 02:58:15 2013 -0300

    Alter users resource to be singular, require login on all actions that change account data and force changes to be over current_user

commit 3d39394d6180888be07855a14cfe028cdc47f5df
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 02:17:15 2013 -0300

    Add integration tests for login / logout process

commit cb04fbf18dd66f99870b2aa76b75b94b2245b497
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 02:16:24 2013 -0300

    Add links for login, logout and signup on layout

commit 4f0e896e2843fe993392e5ff754e0901fa3640b1
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 02:12:46 2013 -0300

    Add Users and Sessions controller

commit 919499e987aa874836965776e684d2f5b385e3c5
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 02:10:10 2013 -0300

    Remove default index.html

commit 15e1077f87bdf003876829cbdcc856f9bda591c7
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 00:02:08 2013 -0300

    Add basic user model

commit 8dce4c66bf523a42ca43c72616fba7bb5cbec849
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 00:01:13 2013 -0300

    Add users table

commit cbcb64226d48b9c74c2862a454c2d6a6a4455ef0
Author: Everton J. Carpes <everton.carpes@gmail.com>
Date:   Mon Feb 18 00:00:54 2013 -0300

    Add bcrypt gem used by has_secure_password