add windows_chef_zero TK provisioner

This will be point at the released version once a release is pushed to
RubyGems.org.
Add Windows 7 and 8.1 to `.kitchen.yml` platforms
Add Windows support to `_user` recipe
Add Windows support to `_git` recipe
Add Windows support to `_packaging` recipe
Create Windows provider for the `ruby_install` resource
Add Windows support to the `ruby_gem` resource
Add cross-platform support to `_ruby` recipe
Ensure Unix-y recipes are no-op on Windows
Perform user creation early in config process

We'll also move all .dotfile creation into the associated recipes.
Ensure recipes declare dep on chef-sugar
Remove `macosx-10.9` from TK platforms

Due to errors in the DSCL User provider `kitchen verify` does not pass.
We'll just remove the platform for the time being until we get time to
merge fixes into Chef core.
Fix spec coverage based on recent refactors
Add a group for the build user
Add integration test coverage for generated .dotfiles