Changes and clarifications for default attribute declaration

* Change how default attributes are set in `attributes/default.rb` file for
  consistency with how they are set when they are cleared in
  `simple_iptables` recipe

* Clarify in the README that the `simple_iptables` recipe needs to be included
  before any of the resources provided by the cookbook are used

The changes in this version resolve #37
Merge pull request #44 from rtkrruvinskiy/default_attributes

Changes and clarifications for default attribute declaration