chef-12 attribute changes

- adding rm, rm_default, rm_normal, rm_override APIs
- adding default!, normal! and override! APIs
- changing force_default! and force_override! APIs
fix autovivification
fix rspec truthy things
add md files for chef-12 attr syntax
Merge pull request #2097 from opscode/lcg/chef-12-attr

Lcg/chef 12 attr