New beautiful style + user_access_level + registration options

I have create a new style that I hope you will appreciate.
![php-login](https://cloud.githubusercontent.com/assets/5228432/2852514/5cdb4126-d136-11e3-802e-c3ade2455cb5.png)

Based on the (Pull request of jessejcollins)[https://github.com/devplanete/php-login-advanced/pull/12], I have:
- create a new field `user_access_level` in the users table. This field can have any value from 0 to 255 and you can use it to manage user right (example: 255 = administrator, 200 = moderator, 100 = top members, 50 = members...). The value is stored into  `$_SESSION['user_access_level']`

And add 2 options in the config file:
- `ALLOW_USER_REGISTRATION`: If set to false, new users cannot registred and registration link is not displayed.
- `ALLOW_ADMIN_TO_REGISTER_NEW_USER`: Administrator (user with an access level of 255) can create new users.