Introduce property instead of attr

In some situation, attr method calls Module#attr, not Serverspec::Helper::Attributes.
Show deprecated warning when using attr and attr_set
Replace attr to property used in serverspec/helper/detect_os.rb
Remove spec/helper/attributes_spec.rb because it's not needed anymore
Merge pull request #293 from serverspec/introduce-property-instead-of-attr

Introduce property instead of attr